Computer randomly shuts down during games!!

Hey guys, Ive been trying to fix this problem for a few days now. I just built a new computer and when ever I play games it will just randomly shutdown every once in a while. Heres all the things I've already tried:
I've tried reinstalling every driver imaginable.
I've monitored the CPU temps and they never go above 50.
I've tryed different sound and graphics cards and it still happens
I've downloaded windows updates
I've run virus scans.
I've made sure nothing is shorting out in my case.
I've run every part of tuneup utilities 2006.
I've tried just about all the setting in my bios, I don't overclock.
I used the RAM, sound card, and video card in my other computer and had no problems.
I did a repair install on windows.
I ran a diognostics on my harddrive. Every things fine.

Ive tried all these things and my copmuter still just shuts down in games. the only thing I could think of is maybe somethings defective. I don't know, I'm getting really desparate. PLEASE HELP ME OUT!!!

Here are my full specs:
Amd x2 4200+
asus a8n sli
evga 7800gt
cooler master wave master.
sony dual layer dvd drive
Antec neo HE 550 power supply
pqi dual channel ddr400 RAM
74gb western digital raptor HDD
Creative X-fi sound card xtrememusic

The Games section is meant for discussing games and gaming. I think you have better luck finding an anwser in the Hardware -> General Homebuilt section so I moved this topic over here for you.

Also, does your computer have any other problems than games? Music, internet browsing, multitasking etc... Because shutting down in JUST games is a little curious.

WHen you say "shuts down" do you mean the game crashes back to the desktop, or the entire computer just powers itself off instantly?

If the latter, I'd slave in another new PS at least in the 500-550 watt range...

(Is your 7800 fed its aux pwr input via it's own molex pwr input, not via a Y adapter?)

Even Antec PSUs can develop problems....