The two causes i can think of are cooling and power. You said that the cooling is fine, so thats probably not the issue. Maybe its the power supply?
are you overloading the 12v rail? If your PSu has two rails, try to balance them out. When did you build your system?
Im guessing its a PSu problem, but it could be a wiring, MOBO or cooling problem. Give me the details of the PSU