The problem is that the source of this problem could be anything from a software/driver issue to a problem with the motherboards/RAM/video card. If you don't have huge amounts of data to back up, I'd say that the quickest solution assuming that the problem is on the software end, is to reformat your hard disk and do a clean install of windows, and install updated drivers for all your hardware.

That should take care of it. If the problem returns after the clean install, at least the problem source will be isolated to be the hardware level, and we can proceed from there.

If you have any way to monitor the cpu / video card temperature, that would be a great help in determining if there's any overheating involved.

Now go fix it and come back to the game bloody wanker mate

test your power supply, your shits only resetting under load, and from your description its getting worse..

thats my opinion anyway :)

Agree w/Reb&Rok- You'll want to take it in to the shop in that case. Tell them the symptoms and they'll hopefully figure out that it's your PSU or vid card/processor overheating.

Last question: does your computer Blue Screen before shutting off? Or does it just plain shut down, black screen?

Blue screen could point to corrupt driver/disk (could still be PSU/temp probs) in which case you'd want to try a wipe/reinstall before moving towards hardware.