Random Poweroffs

Pretty sure at this point it is a hardware issue, but still not 100% certain, so here we go.

System Specs:
MSI K8N Neo2 Platinum
AMD64 Socket 939 3500+
OCZ PC-3200 DDR 1GB Dual Channel Kit (OCZ4001024ELDC-K)
EVGA NVIDIA 6800GT
CoolMax 400W PS
2 Hard Drives and 1 Optical Drive

Computer has been experiencing random hard poweroffs. These poweroffs are to the point where pushing the power button on the case of the computer will not start the computer up again, and I have to pull the plug on the power supply, power cycle the power supply with the cord out, and then replug back into the power supply before the computer will start. At first I thought the problems were software related, as all my poweroffs occured while playing Hitman: Blood Money. Then, I started getting some while doing normal internet surfing, so I ruled out software.

Checked temps of all things on the computer, seems fine. As closely as I can tell (monitoring the temperatures as much as possible while not knowing when the computer will shutdown on me) the temps stay within operating range. CPU idles at 43, load at around 57 (while gaming). GPU idles at 52, load at around 65 (also while gaming). Dusted out the case and all components. Checked the event viewer to see if anything errors out right before the shutdown, or if its a really weird BSOD, but nothing. Nothing is overclocked on the system. Timings on the RAM are slightly tighter than usual, but I put everything back to stock and it still shuts down. Voltages are close to the edge of the tolerances, SpeedFan reports my +12v at 12.71v and my 3.3v at around 3.19v. At this point I'm either leaning towards the power supply acting up, or the video card acting up, as I'll get the crash more quickly if I use the video card more (I think, not conclusive evidence yet on that front).

Wondering what else I should do to narrow down the culprit. Thanks a lot for the help.

EDIT:

Forgot to mention that this only started happening when I came back home for the summer, had no problems while at college. So a third possible culprit may be the wiring in my house, but I'm not too sure.

 

Ok, so I went and took apart the whole computer. Reseated everything, cleaned out the dust, etc, etc. Seems to work....longer? But it still will shut off on me completely. Ran Memtest overnight and the RAM is fine (went through all 12 tests a couple times). The shutdowns are much more specified now I think. I only experienced it yesterday while gaming and specifically during load areas.