Probably a heating issue. How old is the computer?
Get a can of compressed air and clean out the inside of your case with it. It might also be a good idea to reseat your heatsink with some new thermal grease. Don't forget to keep yourself grounded.
THNX. Can/will do on the cleaning. Not familiar with heatsink.
Big metal thing with a fan on top of it beside the ram at the top of the motherboard. You can find instructions online and grease in any good computer store for like $10 or less. If you don't feel comfortable with that though just give it an extra good blow between the metal fins to get rid of any insulating dust. If it doesn't solve your issue, let me know and we'll see if we can get some more details about the problem but it does sound like a heating issue.