When you say the computer turns off and you have to reboot, what do you mean? Can you give a little more information?
I've only had power failure/turning on issues with two computers. One was using a power supply that had one of the 20 pin ATX cables missing entirely. Fans would go when the power button was held down, but nothing would work. A new power supply did the trick here. The other case was where one CPU pin had broken and the computer would turn on (but not post) for 2 seconds or so, turn off for a second, and repeat until the power plug was pulled. The motherboard had to be RMA'd (the new one resolved the problem).