Could be a few things really, but the most likely is some sort of power supply/overheating issue. Check that your PSU is adequate for the system you have. What are your specs, and how long has it been doing this since you've had the system?
Also, make sure the system isn't getting too hot, try leaving one of the side panels off for a few hours and see if keeps doing it then.
Let us know how you get on with this and what your specs are.