Before even reading Hopkirk's suggestion my mind had also turned to overheating. Blocked vents or a failed fan might be the cause of the problem. (My friend had a similar problem many years ago. When I went to her flat to investigate the problem, I discovered that she'd placed a bar towel on top of her desktop PC, so that she could put her coffee mug on it. Not realising the importance of keeping a PC's processor cool, she'd allowed the towel to hang down at the back of unit, thus obstructing its vents. Removing the bar towel fixed the problem for her!)