Computer shuts down when playing games

My PSU blew a couple of weeks ago and I bought a new one (it's probably crap too, as i was looking to save money, so bought Q-Tec 450Watt one). I also bought a new MOBO (MI K7N2 Delta Series MS-6570 (v1.X).

I have 1Gb of PC2700 RAM
AMD AthlonXP 2600+ (FSB333)
Nvidia Gforce 6800GT
Audigy ZS
80 & 60GB HDD

Anyway, everytime I try to play doom3,farcry or run 3dmark03 it starts fine and runs very nice (when running precaches time demo on doom3 at 1024x768, ultra high details, x4 AA Vsync off, it does 51.1 fps, or my farcry everything at max never falls under 35fps, and most of the time hangs around 40-60fps) but after running any of those for a while the machine switches off, and not like someone forcing a shut down on it, it's like someone pulls a plug out, and the only way to start it up is to unplug it, wait a few seconds, plug it back in and it'll start just fine without any hicups and no sign of anything going wrong.

I just put it together last night and installed windowsXP home on it. All the drivers seem to be there, and it runs fine if i don't play any games or run benchmarking tools.

Does anybody have any idea what's going on?

Please help.

Definitely sounds like the PSU being overloaded. Q-Tec PSUs aren't too bad, I've used a couple. At least you know it trips off when overloaded - the REALLY cheap ones just blow up and can take other components with them ..

Use something like motherboard monitor, set to log all your voltages to a file every couple of seconds to see if they dip just before crashing.

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Epox 8RDA+ V1.1 w/ Custom NB HS
XP1700+ @200x10 (~2Ghz), 1.4 Vcore
2x256Mb Corsair PC3200LL/1x512Mb Corsair XMS PC4000 2.5-3-3-7
Sapphire 9800Pro (VGA Silencer Rev3) @418/742

I had a little look and did a littl ebit of testing and came ot a conclusion that the system power goes down when I load my Gcard to the max, but when the power does go down it's not gone, my LAN LED is still on when it goes down. Is it possible that the system overheats and my mother board cuts the power as a safety mechanism? I the monitor app that came with my mobo it shows that system temperature can easyly get upto 50C and the CPU temp goes upto 65C with my Gcards temp reaching 85C+ easily. And the system runs really nice when not loading my gcard too much.

So is it possible that it's an overheating problem?