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After getting everything running for him last night, he took his computer home, and… Back to square one. Behaving the same as when he brought it for me to look at, though no changes were made; I can only assume something came loose in transit, but we see where assumptions have gotten me so far in the case of the cursed computer…
Transport has disabled computers from time immemorial. Connections are so easy to vibrate or jar loose.
I never count my troubleshooting as done until the device is working correctly after being re-installed in its final spot.
At this point I’m gonna suggest you somehow got a bad batch of mobos. Or in the alternative, a power supply that’ running off-voltage and cooking parts.
Thank YOU for stepping back in to let us know.
Before I retired, the thing I disliked most when I did tech support was continuing to research a problem only to hear “oh I didn’t want to bother you just to say it started working”…
Maybe memory modules were not clicked in properly after swap testing?
Have you been through the BIOS/UEFI and made certain things are as they should be in there? Disabled onboard graphics? Set memory timings correctly (using EXPO or XMP if your RAM supports that or set manually to conservative settings)?
Fiddle with that. Swap memory to be sure. Is the RAM in the correct slots? (can’t be in just any slot, install in pairs ideally with matched RAM…check manual for proper slots with what you have)
Is the RAM compatible with the motherboard? DDR4, DDR5, timings? Double check. XMP/EXPO are the simple way to do it if it is supported by the RAM and mobo. Since this is an AMD mobo I guess EXPO is what you want.
Otherwise, time for a new mobo. Seriously. Unless you (or whoever) didn’t properly seat/connect bits it seems this is the memory or that mobo.
You’ve tested and tested. Time for a new, better mobo I think. It seems your problem is the memory or the mobo. At least get a mobo that will tell you where things are going wrong. (my guess it is the RAM)
That’s the most satisfying part of building a PC…getting that click when putting in new memory. 
Yeah that is a good one.
He found another memory module at home, swapped that in, and everything worked again…
Amen, LSLGuy.
I’ve been a services manager for several companies who installed and serviced fairly extensive security, video, and fire alarm systems, as well as for companies working on desktop computers. I’ve always believed that people are either born troubleshooters…or, they’re not.
I’ve gone blue in the face explaining that “there’s no such thing as magic…there’s a real cause for the problem” and “if you didn’t measure it, it’s not true.” One frequent shortcoming in technicians was that, once they had identified a specific problem in a system they repaired months ago, they would begin EVERY job by trying the same fix, even if the symptoms were completely different.
The other two maddening things about the techs were that many of them never learned to use their (relatively simple) test equipment and they were not inclined to replace a suspicious device with a known-good device to see what happened. (And we made sure they had adequate good stock on their trucks.) They’d call me at the office and say, “Camera # 3 is not working.” I’d ask if they had tried substituting a used, but working camera, in place of camera #3. They had not, but they would. When everything works fine, I’d ask, “So, what does that tell you?” They’d hum and haw until I’d explain, “It means the cable and power supply are good and that the old camera was bad.”
It took me years to finally figure out that you just couldn’t teach some people to troubleshoot logically and efficiently. They either had it…or they didn’t.
OTOH, sometimes bad troubleshooters did the most beautiful installation work.
Hmm. That makes me wonder if the entire kit was from a bad batch. I hope you took advantage of the warranty and replaced it.
I had possibly a dumber story. I got all my parts, and I tried it, and it didn’t work. I find out if the mobo had the required BIOS, and it did, and the other parts worked in other computers, so I try returning the memory, and go for one explicitly known to work with my mobo and CPU combo. I get new memory, and it still doesn’t work.
Turns out, it was the monitor! Or, more accurately, the device I used to convert between two plugs. It worked fine on another computer, so I never though to try one of the other monitors I still had.