I'm guessing it's the PSU, and it's not a big deal...I'm just curious if anyone else out there has had this happen! Once the machine posts, it's fine, it's just that if I have to do a complete shutdown, then I have to go in the case, take out the 12v 4-pin, start, and then reinsert it again...LOL. I've had this system for 5 years, currently building a much more modern quadcore.
This is almost certainly the power supply giving up. Try substituting the power supply. If you are very unlucky then the problem is with the motherboard, look at the capacitors on the motherboard are they bulging?