Iwould try a different PSU, even if it's outside the case, just take the one of your desktop that you know that works and try it and see. Help eliminate or confirm it's a PSU. If it's not that, then it's probably the MB. usually the problem you describe happens on old boards with a dying capacitor, I see it a lot. But not much on new boards. Either way, 10 mins and a star head and you can test if it's PSU or the MB.