and then, if it fails, take it out and leave it out, with the power cord disconnected, for abut 30 mins.

Replace it, press the cuda ONCE, and fire it up.

Google for power supply pinouts for that model. Beware, nearly all of the G4 models had different power supplies and some had different ones in the same model line!

I just replaced a G4 MDD power supply at a cost of $164 plus shipping.