I have had similar issues where it would power on and power off after a couple of seconds, Happened on two power supply's. So i knew it was probably something with the motherboard, Happened to be a loose heatsink on the north/southbridge since I was rough handling the motherboard. Finally got it to work after fixing the heatsink mounting on the north/south port bridges
This should fix your problem. Heatsink must be fitted well because if not it might harm your units. And I think it is the reason why your psu if failing.