I would say the CPU is probably not what's bad. You're able to boot into Windows and run GPU-Z and run VMT without crashes. Your pre-POST crashes could indicate a bad CPU, but I don't think so. A bad CPU causes massive havoc at almost all times, and the behaviour is usually noticed pre-POST, immediately after POST, or *definitely* as Windows boots/loads drivers.
You could test system RAM if you wanted using memtest86+
(download the pre-built ISO, burn it, boot it -- nothing else you need to do). Let it run until "Pass xx%" has exceeded 100% or has wrapped back to 0%. This usually takes a few hours. What you're looking for is something like this
. If this tool locks up hard then the problem probably isn't RAM-related but rather "something else" (PSU, voltages, motherboard, etc. -- but not the GPU. GPU isn't heavily used in memtest86+, obviously).
PSU testing is only helpful is you can test the PSU while it's in use, i.e. an "inline" or "passive" test. I have one of these
which is not an inline tester (this device has said "OK!" many times with PSUs which were absolutely 100% bad when put under even light load). I have a Fluke MM myself but I am completely/entirely afraid of anything pertaining to electricity (really!) so with PSUs I tend to just buy another and re-test.
If I had to take a wild guess at the mess of a system? It'd be that you have two actual problems:
1. Possibly bad motherboard. Northbridge or southbridge, VRMs, or some other anomaly (cracked traces somewhere between layers, etc.). This is really hard to diagnose, and often manifests itself as the system just flat out locking up hard. Since VRMs are involved, and those are what provide voltage to your GPU, its possible that a busted VRM could cause OCCT to report errors (since your video card is powered off the AGP bus, not off the PSU directly)
2. Your GPU is by far running way too hot. Rather than futz with this I would recommend just buying another video card. This is hard to do since you're limited to AGP (good luck finding AGP cards these days -- I'm sure some are still made but ha!).
Let me ask you something very bluntly: are you willing to replace the parts in this system with something more recent, assuming you can do so on a budget with some help (from me)? I have hardware right now which I could send you (mostly free of charge -- some you would need to pay for however) that could build you a reliable replacement system (Socket 775, PCI Express, etc.). Specifically: motherboard, CPU, and video card. The board uses DDR3 RAM (one of the few Socket 775s which do) and I don't have any of that for you, but it's *super* cheap right now. I have the details typed out and ready to so just say yes/no.--
Making life hard for others since 1977.
I speak for myself and not my employer/affiliates of my employer.