I think I would have bought a new PSU when this first happened, particularly so if it occurred without the intermediary UPS.
At the very least, I suggest taking the PSU out of that system and momentarily (all you need is a minute or two) switch it to another system. The alternate method (switching a PSU in to your system) would also be good as a way of confirming/refuting any doubts you might have re the rest of the system.
Testing a PSU by switching is normally quick and easy, which is the way I like diagnosing
Some of the other members/visitors can tell you alternative ways of checking your PSU.