No. I was rounding down, as I forgot you could get data to the tenths place. The number is the percentage of time that the CPU is dealing with interrupts. 0.1% is fine.
That’s actually one of the ways to check and see if you have a hardware problem. An interrupt happens when CPU has to interrupt what it’s doing with software to deal with a message sent by the hardware. It’s normal to happen occasionally, but it should be relatively quick. A higher percentage tends to mean the interrupts are lasting too long, meaning the hardware is having trouble talking to the CPU. This can be because of the driver (software) or because there is something wrong with the hardware itself.
I agree with you that a memory leak does not seem to be your problem. But there are a few other software problems to check if you want to completely eliminate a software cause.
The first is one that I hesitate to mention, since its got a bad reputation of being suggested by people who don’t know anything about computers. But it’s relatively easy the possibility needs to be eliminated. Do a malware scan. Slowdowns can be caused by malware stealing CPU or memory resources, and many hide that fact from tools like task manager. Usually it’s a bit more obvious than your symptoms, but checking won’t hurt.
Also, you are using Firefox, and Firefox has freezing problems sometimes. It probably would do you some good to refresh Firefox. You will lose your extensions and download history, but not much else. (And you can get them back if you really want to–Firefox makes a backup.)
If you want to try it, go to Firefox > Help > Troubleshooting Information and click the “Reset Firefox” button. A backup copy of your profile will be placed on your desktop. If you don’t want to mess with that, you can also always make a system restore point before you start it all.
To be sure it’s hardware, you also need to take one more step (if the above doesn’t fix the problem.) And that is, yes, reinstalling Windows 8 fresh. You need to eliminate problems caused by the in-place upgrade. This will also allow you to reinstall software one piece at a time to see if any of that caused the problem. And it will also eliminate any malware that you couldn’t get rid of normally or couldn’t detect.
After going that far, then I would pretty much assume it was hardware. And, unfortunately, hardware problems are not my specialty, so I’ll defer to people who seem to know more. If you haven’t already, definitely try the heatsink thing.