It would help if you supplied more info on your computer.
Laptop or desktop?
What video system do you have? (on-board, discrete card, etc.)
In any case, it sounds like overheating. If it's a laptop, check that the air vents are clear of dust and that the cooling fan is running properly. The fan should basically be a blur, not stopped or only turning slowly.
If it's a desktop, open it up (usually by taking the side off) and blow the dust out of it. Make sure that all heatsinks are clear of dust and that all fans are running properly.