I’d be willing to bet that Malwarebytes is the problem.
Windows is starting up some process when the system is idle, Malwarebytes doesn’t like it, and the two are fighting (and possibly overheating the CPUs) until the system freezes.
The solution will be to 1) Check the settings of Malwarebytes and see if you can turn off most of the features, especially heuristic analysis. 2) If that doesn’t work, deactivate Malwarebytes and switch to Windows Defender.
Well, I shut down Malwarebytes yesterday, and no overnight crash. However, it was still shut down when I was just in a videoconference using UberConference on Chrome when I had another freeze crash. (Maybe I should write a sequel to Snow Crash.) Although some crashes have occurred when not doing this, a large proportion of them occurred while using this. It is browser-based. When this all started I thought it might be compatibility issue with my video card so I removed it (still removed) and still had crashes. Then I removed my web cam and still have crashes.
I have also tried using UberConference on Edge and also had crashes. I had two crashes in a row using Microsoft Teams. I wonder if there is some underlying hardware or or system software that comes into play when these videoconferences run. UberConference is browser-based but has a Chrome plug-if you want to share your screen. During my crashes I was not sharing my screen. Teams has its own application.
I haven’t read through the whole thread, but two things jumped out at me: “built computer myself” and “second monitor”.
While I highly suspect this is NOT your issue: about 19 years ago we purchased a computer from a co-worker. He’d built it himself. We brought it home, set it up with our own monitor (this was a CRT monitor)… and the damn thing would reboot, go for a bit, then freeze. We took it back to the coworker’s place and it worked fine.
We began to think our house had bad feng shui or something.
He replaced the hard drive - and it seemed to work OK.
Then we moved it from the floor, up to the desktop - and it began to freeze again (note: we had originally set it on top of the desk when we first tried it out).
The culprit? A combination of bad shielding on the case, and interference from the CRT monitor. When we set it back on the floor… it worked fine.
So we replaced the monitor and all was well.
That self-same monitor, a few years later, suddenly put itself into mirror image mode - and let me tell you, trouble shooting that… in reverse… was a special kind of fun. But that’s a tale for another day.
That’s an interesting story, and my main takeaway is that the answer is not always obvious. I am not sure of what shielding is even on a case. It’s all metal and grounded so I’m not sure how it shielding would be an issue. In any case I used a commercially manufactured case. And as for CRT monitors–I have a monochrome one in my basement connected to an IBM AT clone with an 80MB hard drive and 12MB of RAM. That computer never crashed. 
At any rate I am unplugging the second monitor to see what happens.
If removing things isn’t stopping the crashes, try adding stuff, and see if you can make it crash.
What I mean is, when things are otherwise idle, run something that pushes the CPUs and gets them good and hot. Then run something that tests the memory, then the disk, etc. I’m not sure what to use on Windows, but on Linux I’d start with stress-ng.
If, for example, getting the CPUs hot consistently causes a crash, then that gives you lots of information. If you can run the CPUs at full blast for hours without crashes, then the whole CPU usage thing might not mean much.
Memtest86, either the free + version or the commercial (but free to use) one from PassMark will let you stress the CPUs and memory free of any interference from Windows. If your system freezes while running those then it is probably a hardware problem, and isn’t the hard drives. (An immediate freeze when the tests start is probably a compatibility issue, and can’t be meaningfully interpreted.)
Do you have a decently-sized power supply? I used to have intermittent/regular crashes (freezes) until one day, my power supply just gave out (with a loud ‘pop’). Installed a new one, haven’t had problems since.
Also, if things get that hot regularly, maybe install an additional case fan if there’s room—I’ve found that getting a good airflow through a case with two fans, one blowing in and the other out, can do wonders for heat related problems.
I have a 1KW power supply, which should exceed the needs of this system.
The case has four fans.
Go here and ask. Some VERY knowledgeable people there eager to help. I’ve never had a problem they couldn’t fix, never.
I have been running this today. The computer locked up on pass 1/4 during the Modulo 20 test. The pass was 75% complete and the test was 54% complete. Then I rebooted and it froze again on pass 2/4, also during the Modulo 20 test. The pass was 75% complete and the test was 82% complete.
It did report any errors, the computer just froze. Does this suggest memory failure?
Hmm, suggests something other than the OS or one of its drivers at least.
At the DC I worked at, when memtest86 would freeze or reboot a system that had been having problems, we’d swap the RAM first, just because it was the easiest to replace. If it continued after that, we’d try swapping the proc(s) if spare ones were available. If it continued after that, we’d swap the board. Never had one continue to do it after swapping the board.
Memtest86’s page on freezes/lockups: MemTest86 - Freezing and Lockups
Correction
Based on that I’d run several more runs and see if it fails more than once at the same spot. If so, that leans towards one bad memory stick.
If failures keep happening but at random places you might try swapping the RAM sticks after cleaning the sockets as best you can. That might move the problem to a different spot or stop it all together.
If you can eventually make every failure happen in just one stick, replace that stick. If not, I bow to @scabpicker’s suggestions for the next move.
All along here I’m assuming your main goal is to avoid spending $ and your time spent fiddling with it is free. If that’s false, just buy / build a new computer. All too often once gremlins start you may be able to put them into dormancy, but they don’t stay dead.
That is false 
and I have been looking for a new mobo/proc/mem combo online for the last half hour.
The current mobo was installed in 2016 and the CPU and memory are from 2013. So I am starting to think this is a good time to upgrade.
FWIW I had been living with a 2010 laptop as my “desktop” machine with an external keyboard, mouse, & screen. It was increasingly a terrible frustrating experience.
About 3 weeks ago I got a brand new state of the art super tablet/laptop. What a GIGANTIC relief that was. I had not realized just how much BS, kludge, & bad performance I’d been putting up with. I totally vote Go For It! on a new machine.
Moving in was a minor PITA but I learned something you might find valuable.
As a former IT guy I had lots of little tweaks in my old system. i.e. custom environment variables I’d had defined on every new machine since Windows 3.1. And my “standard” layout of main folders on the hard drive long predated Win95’s “My Documents”, etc.
I took a hard look at how much of that tweak was truly necessary and how much was me just trying to retain backcompat with my younger self. And I took a look at what I’d really have to change to go with the current flow of cloud-oriented, always online, etc. It turned out to be mostly a matter of ditching my 30-something self’s overweening and now decades-stale pride, and nothing much of any real value in 2020.
Bottom line: I ditched about 90% of my tweaks as obsolete, no longer need to fight “my” folder organization vs. “their” standard, and life in the box is just much easier than it was.
You mentioned up-thread having lots of customizations. E.g. if you’re running a custom desktop to be XP-reminiscent, now’s probably a decent time to rethink carrying on that hallowed tradition.
Something to consider.
I hear you but that’s not really what I’m doing. These are things like turning on visibility of file extensions in Windows Explorer, setting up my third-party screen saver, importing macro customizations to Office ribbons.
The other thing I need to figure out is if I can reuse my Windows license. I originally bought Windows 7 on this rig (two CPUs ago), and later took advantage of a limited-time free upgrade to Windows 10. I don’t know if I can use that key to install straight to Windows 10 now.
I’m glad your looking at a new system. If Memtest86 is freezing at random places that almost definitely means some sort of hardware problem. It might be a compatibility issue, but those seem to freeze immediately. In my experience, Memtest86 either works perfectly or freezes randomly. Finding errors means that memory is broken enough to generate errors, but not so broken that it causes crashes.
You probably can install straight to Windows 10, but I’m not absolutely sure that works in every case. I had two recent experiences that suggests it is possible. Recently I was configuring a computer donated to a non-profit. It has a Windows 7 Pro sticker on it. I installed Windows 10 Pro, and typed in the product key directly from the 7 sticker, and 10 accepted the license without issue.
In the second case I have a work laptops that came with Windows 7 Pro and the embedded license (not a sticker). Usually I use our enterprise license on them, but my install USB had the Windows 10 Pro image on it from the previous paragraph. Windows 10 Pro again accepted the old Windows 7 Pro license without any problem.
Once again, thanks everyone for all the help offered here. Because this pointed to a memory problem I just decided to rebuild with no further diagnosis. The board is four years old, and the CPU and memory are seven years old. I so I deserved an upgrade.
I rebuilt it yesterday–new mobo, CPU (i5-10400), and memory for under $500. The hardware assembly went very smoothly but I had fits getting Windows installed. I think it was due to an SSD boot drive formatted with MBR where the newer systems demand GPT (my BIOS does not have an option to select “Legacy”). When all was said and done I had to reformat and reallocate the drive and do a clean install. And within a couple of hours got a BSOD. 
The primary suspect was an incompatible driver for the Intel Ethernet. I loaded a generic driver and everything has been fine since then.
I reinstalled Microsoft Office 365 and was able to recover my macros and ribbon customizations without too much trouble. I also had to configure the startup folders and other options the way I wanted them. But Microsoft automatically installed the 64-bit version without asking me what I wanted. I have always used the 32-bit version even on 64-bit machines (the conventional wisdom has been don’t install the 64-bit version unless you really have to), so this fucked up some of my VBA. I have still not fully reconstructed my Outlook configuration. I use four email accounts regularly and two of them are POP accounts so I still need to migrate my old .pst files to keep my mail. (The other two are Microsoft Exchange and Gmail, which both sync to the server.)
I had to reinstall and configure my iDrive backups. Fortunately they presciently have this scenario in mind, and gave me the option to replace another computer with this one.
Dropbox is more of a problem. When I first installed it a while back, it set up a Dropbox folder and you used it however you wanted. Now when you install Dropbox it hijacks your Documents folder. I deliberately kept things in the Documents folder that I did not want replicated in Dropbox and now it just vacuums everything up. I’m still trying to figure that one out.
I’ve installed 22 other apps with few problems. Photoshop Elements complained that I was using a license key that was already used. It took almost a half hour chat with a rep to get that resolved. The same thing happened with Microsoft Project but Microsoft has had an automated way to deal with this for years over the phone. It’s rather tedious–you repeat about 40 or so digits from your screen, then in return they give you another 40 or so to type in. Then it works. I suspect that they do this to prevent fraudsters from doing this in bulk but make it trivial for the average schmoe like me.
There’s another product that I have a lifetime license for–but I am still searching for the license key.
In some cases that might true but I’ll be screwing around getting everything how I want it for the rest of the week.
I have about another 30 things to install but they’re low priority (like Filezilla). I have Steam so it will install my games for me, but I’m not a huge gamer. My first priority was to be ready for work tomorrow morning.
The only serious casualty is that my Logitech headset isn’t detected, and now it isn’t detected by my laptop either, which already had drivers for it. It may be a coincidence–it may have just reached the end of its life. It is, after all, 16 years old. But it has great performance. I wonder if something about connecting to the new computer could have fried it.
Still playing whack-a-mole with applications that give pop-up Windows notifications and sound for events that I don’t care about.
The “Don’t use 64-bit Office unless needed” was excellent advice back when lots of people had 32-bit add-ins for Office and none of those had been ported to 64-bit yet. Like for the first couple years after 2005 when WinXP/64 came out.
I’m no longer in the biz, but I suspect the conventional wisdom nowadays is “The less 32-bit decades-obsolete crap on your machine, the more stable & serviceable it’ll be.”
Check out https://ninite.com/
Install all of your freeware (like FileZilla) from one place in one step.
You can thank me later.