possible hx620 problem?

i've been experiencing some sort of pc problem, namely, it will shut itsself off randomly. it first started when i was playing a game, company of heroes, but after it would happen randomly. browsing the net, just sitting there doing nothing. i took it to my friends house, he ran a bunch of stress test/benchmarking stuff to see if it was overheating or what, but he could not get it to turn off on him, so i took it back, and also couldn't get it to turn off until today, after playing crysis for about 2 hours. the computer will always turn on after, which leads me to believe it's not overheating the power supply, because whenever that's been the case for me it takes it a while before it will turn back on. the cpu isn't overheating, it only gets up to about 45 under load. i've ran memtest on my memory and haven't had any errors there. so i'm wondering could something be drawing too much power on the supply? i'm pretty sure the hx 620 is plenty more power than my set up needs... perhaps i'm just cursed by the computer gods? i don't know. any tips/help would be appreciated.

automatic shut off usually connected to failed drive(s), or driver(s) and not the PSU by itself, although the latter is not 100% true, i think you should try your PSU on another PC. (correct me if i'm wrong).