Power supply problem! Need help quick

[Quad]

After shutting down the computer and leaving it off for a few minutes, it won't turn back on when pushing the power button

I have an atx power supply. it's not the power switch, because when i short the pins for power on the mobo, it still doesn't turn on. However, when i unplug the power cord for a few minutes, then plug it back in, the power supply turns on the instant i plug the cord back in, and the computer boots.

update: if i short out the green and black wires on the motherboard connector, then plug the power cord into the psu, the psu will turn on. could this be a mobo problem?

any ideas what's happening? thx in advance!