HELP: TROUBLESHOOT - PC SWITCHES OFF WHILE GAMING

Hi, my PC is switching itself off after 10-15 mins of graphic intensive gaming ie Med 2 or STALKER. It plays Rome 2 and IL-1946 for hours and hours on end. Vista tells me it's a hardware error - CPU, RAM, MOBO, or PSU, have you any idea how to begin troubleshooitbg this? i don't have anyone local i can interchange parts with to test them, my rig is moRe than up to playing the lastest games at good settings.

E4300 at stock
P5NE-SLI
2 GIG G-SKILL (BQ) RAM 6400 800MHZ
EVGA 7950GT KO SUP
X-CLIO GREATPOWER 750W PSU (MOD)

Any ideas?

I'm going to do some more testing with Oblivion and report back.

The airflow is very good, front to back, i have 3 120MM fans, front, side panel, and exhaust, plus a large PSU fan.

I had forgotten a very small OC i had on my E4300, just up from 1.80 to 1.90GHZ, set up with a 5% auto OC - it went back to 1.80 and Stalker played great for 40 minutes before i quit the game, hopefully that has sorted it out.

Thanks for the reply, hopefully my PC is back being solid again.