Unexpected shutdowns during gameplay

Hi, I'm new here so sorry if i have posted this in the wrong place, but i have searched the whole forum and cannot find a similar problem.

I recently bought a new custom built pc with the following spec:

Intel Core2Quad Q6600 clocked at 3.00Ghz
Arctic Cooling Freezer Pro 7 CPU cooler
8GB Corsair XMS2 DDR2, 800Mhz
500GB SATA II
DVD RW
DVD ROM
ASUS P5N-T Deluxe
Nvidia GTX280

Running vista home premium 64 bit.

It has a 700w power supply.

The pc Runs fine and can cope with 3dmark happily.
But when a game is run, ar ound 10 minutes to half an hour in the computer switches off, no warning, just powers down.
It happened on the first day i got it while playing crysis, I tried again and the same problem occured
After a few times it wouldnt switch on at all, I returned it to the man who built it and he tested the power supply, it was dead, so we presumed this was the problem. A new one was fitted and it was tested and 3dmark was run a couple of times. Everything was fine, got it home, used it for a day, then tried to run crysis again, the computer switched off. Rang the man again, he said there was a problem with crysis and vista 64bit and i needed a patch. I tried a patch (dont know if it was the right one) and still had the problem, I then downloaded Codemasters racedriver grid demo, the same thing happens with this.

Any Ideas?
Thanks Dean

Could you tell me the brand and model of that PSU?

Hi, the psu is EZcool, model number 900, or something like that.

I also thought it could be the card overheating, but I ran the game in windowed mode with expertool running alongside showing the temp of the card, once before it switched off it was 102C and another it was around 70C but neither were in the red on the graph. I also manually set the fans to 100% and 3Dmark ran for slightly longer but it still shut down.

I have underclocked the card and switched the fans to full, and it seems to be running fine. But the card will be pulling less power now, so could it still be the psu?