Apparently my power button is on the fritz. Recently, after a cooling system revamp during which some coolant was spilled on the MB (frying my RAM but not, as far as I can tell, landing anywhere near the power button), I began finding that if I didn't push the button in all the way and hear a *click*, the machine would turn on for a few seconds and then shut off.
It didn't seem to be much of an issue, other than irritating. This morning it got so sensitive that if I did *anything* to the button, ie brushed it with a fingertip, pulled up at it when it seemed to get slightly stuck, or just clicked it normally, same thing happened. Power came on for a couple seconds, no picture on the screen, and then it'd shut off.
Luckily my MB has a hardwired power button. I tried that, and same thing. I pulled the top of the case off, kept trying, same result. I finally unplugged the external button. When I did THAT the power came on for a couple seconds then shut off. I let it sit, then hit the hardwired button. After that it was fine.
I guess I need a new external button, as I can't reach the hardwired one when the case top is on. Oy vey...as long as I don't need a new PSU...Anyway, thought I'd share in case anyone else ever encounters a similar issue. It was weird.