Hmm, possibly overheating? Open the case while it is running and make sure all of the fans that should be running are. If everything is fine, grab a can of compressed air and blow the dust out of them (while the comp is off this time). I would suggest downloading TAT or speedfan to check the temps, but your mobo may not have any sensors on it. My Dell only has a sensor on the HDD.
Could also be a PSU problem. Grab a unit from another PC and put it in that one and see if it changes.