This was a really weird ride. Recap, BIOS screen had an error on startup, "Failed to OC CPU." I removed the OC. System would reboot after about an hour (happened multiple times). I started testing individual sticks of RAM. With stick A, system would reboot after about 2 minutes, consistently. With stick B, system would reboot after about 15 minutes, consistently. With stick C, system would reboot after 30-40 minutes. Happened twice in a row.
I started to wonder if dust was the issue since my PC was pretty dusty. Bought some air and dusted it out. With all 3 sticks on RAM in, the system stayed up for a few hours before I shut it off before going to bed.
Next day I ran memtest86 on stick A for 10 hours. 2 errors. Ran memtest86 on stick B for 2.5 hours (6 passes) No errors. Ran memtest on stick C for 7 hours. No errors. Ran memtest on stick A again for 9 passes (4 hours). No errors.
So I can only conclude that the problem was dust. Though I never would have thought dust could cause such an issue. It's not like my PC was absurdly dusty either. It had been about a year since I last dusted it out.
But since I bought some RAM assuming my RAM was the issue, I'm wondering if I can use my new RAM with my old RAM. My old RAM is 1.65v and my new RAM is 1.5v. Is it ok to use DIMMs with mixed voltages?
Edit: Just to clean this issue up, I finally found out that the problem was my UPS! Plugged my PC directly into the wall and all is fine now. I would have thought a UPS would have lasted longer than 1.5 years