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.
Ep, glad to see you come back and tidy up...did want to ask a one day favor, I want to enhance my resume , was hoping you could make me administrator for a day, if so, take me right off since I won't be here to do anything, and don't know the slightest about the board, but it would be nice putting "served administrator osnn", if can do, THANKS