Sounds like one of the power switch wires is loose. If i were you i'd open her up and try testing the wires. I'd give you a diagram pointing to them, but i don't have Bunnie's book with me right now.

I modded my power supply to have a remote switch and i noticed that some of the wires weren't attached very well. The guys over at Xbox-scene would probably be more helpful then us. They really know their shit.