Computer Randomly Shutting Down

I'm only able to play about 2 minutes of a game before my computer shuts itself off. I've tried cod4, tf2 and css, all have the same problem. I just ran memtest and performed a virus scan, no issues there. This problem just started last night, and my temperatures are no higher than they usually are (CPU around 38 C, GPU around 60 C - idle). I find it hard to believe my GPU/CPU temps are getting so high during those 2 minutes of gameplay that the computer has to shut down. At first I figured it was my PSU, but the computer will only shutdown when I'm playing a game.

Specs:

Intel C2D E6600 @ stock speeds/voltage (Zalman 9700 fan)
2GB Corsair XMS2 DDR2 800 (5-5-5-18) @ stock speeds/voltage
EVGA 9800 GX2 @ stock speeds/voltage
OCZ GameXStream 700W PSU
EVGA 680i SLI
Soundblaster X-Fi Titanium

Suggestions?