[skoal2k4]

i had a problem similar to that. It ended up being my video card. The fan on the card had stopped working causing my video card to overheat. Once it got too hot, my pc would just lock up on me and I'd have to restart, but it would just overheat again and lock up. So check and make sure that all your fans are working correctly and that you've cleaned (I just spray with compressed air) the fans and air intakes/exhausts. If the fans are good and everything is free of dust, you might have a bad card... video, sound, etc. I really don't believe this to be software related since you haven't made any changes

Hope this helps!