Another thing you might check is your ATX connector. If the board has been in service for awhile, it's possible that some of the contacts may have started to oxidize, creating higher resistance and possibly fluctuating voltages. Worst case is when some of the pins actually overheat and burn/melt the connector to the mobo.
Pull the main ATX mobo power connector and check for any signs of over heating or burning. If all looks OK, try re-seating it a few times to clean up the connections. Might be worth a look...