You appear to have answered your own question! It's almost certainly an overheating problem.
First check that the vents in your PC housing aren't blocked. (A friend of mine placed a bar towel on top of her PC tower, so that she could put her coffee mug on it. Unfortunately the towel was also draped over the rear vents of the tower and she couldn't work out why her PC was dong odd things!). Then open up the case, identify where the fans are located, and (while keeping your hands and the cat's inquisitive nose well out of the way) power up the PC. The chances are that you'll find a defunct fan. (Whether you do or not, it would also be a good idea to get rid of as much dust as possible - with the PC switched off again! - while you've got the case open).
Chris