My G3 Yosemite and Yikes G4 both have displayed similar symptoms when the PRAM battery has failed... unsure why, but changing the battery fixed the issue. New world Macs do some strange things when the PRAM battery is getting weak. The G3 also at one point refused to boot properly or even power on sometimes with a flat PRAM battery until i had hit the cuda button on the logic board a couple of times and even at one point had to hold in the interrupt or whatever it is button on the front as if to complete a firmware flash before it would boot after sitting for a while.

Not wanting to fill your head with excessive optimism, as MDD's are known to have issues with PSU's as well, however a PRAM battery is a cheap and easy thing to at least rule out, rather than going straight ahead and replacing or repairing a PSU. Best of luck!