Randomly Shutting off

Everything has been running fine since my first build in July, yet of late ive noticed decreased performance in games. This went on for about a week before my rig started powering down with no warning. I don't believe it is a software issue as it powered down in BIOS as well. Any help isolating the problem would be very much appreciated

My rig: e8400

8800GTS

4 gigs DDR2 RAM

500g seagate HD

Thermaltake 600w Purepower PSU

do u smell anything unusual especially coming from the psu? cuz i had that problem be4 and it was my psu frying up. but it may not be the case. many possiblities may cause such problem.

so you checked the temp of both gpu and cpu? here's one thing you can do. i assume u are running 2 sticks of ram in dual channel right? try taking one out and running ur pc with 1 stick and then swap it to see if the problem persists. if it stops then u have a bad stick of ram which could also cause shut offs.

I've got the most recent drivers, but the system shut down in BIOS as well so it couldn't be a software issue. I tried the RAM and both sticks worked. My new theory is my PSU ( the 600 watt Thermaltake Purepower ), is only outputting 18amps on each 12v rail whereas the 8800 GTS says it requires 25amps, any thoughts? keep in mind its been performing fine since July.

Thank for your input by the way guys, i really appreciate it.