It could be a driver issue as in motherboard drivers and not train drivers, maybe the video driver needs updating or card is faulty, does the cpu have a function where when it reaches a certain temp it shuts down, usually to protect it from overheating and can be modified in the bios. Or, you could run a direct x diagnostic, click on start, run,and type dxdiag. Mine shuts down occasionally and it seems to be in the middle of the day when its hot and i've been pushing it a bit hard. Hope this helps a little bit!
Regards
Jazza