ASRock P4i65G

I am not familiar with the Mercury UPS systems, but does the problem occur if it is not used? If not, then it is obviously the UPS, and this should be the first thing you check, just to be sure.

If the problem persists, then the problem would probably be internal, and I would suspect that the power supply is not supplying the proper voltage on the 12V rail. So the next step would be trying a new or known good power supply. With luck that would resolve the issue.

If not, then the next most likely problem would be the motherboard itself, but at that point it is always a good idea to test as many components as possible. Like putting your RAM in another machine, a different video card, etc. it never hurts to remove all non essential parts, like a modem, etc., and unplug any USB devices so that you can just test the really necessary things.