From what I have read, no personal experience, if you press the PMU button more than once in quick succession, the circuit can go into free running oscillations and will thus drain the battery very quickly - but don't quote me on that.
I had, actually still have, a G4 where I had to push the PMU reset button every time to get it to start up.
I just left it running and eventually switched to a Mac Mini. The G4, even with a faster dual CPU was just too slow for what I needed and a lot of software was no longer supported on the old machine.
Unplug your computer and take the battery out, and leave it out overnight. That should clear out everything.
Put the battery back in the next day, plug it in, and pray.
Sounds like a bad power supply though.
Real Macsters don't repair permissions.
Opinions are like toothbrushes. Everybody has one, so there's no need to share. - Red Green
And there is a fellow in Montreal on L-Acadie who may have a used power supply for your Mac.
He works out of his apartment - replaced the screen on a MacBook Pro for me - very reasonable and it works fine.
Replacing a power supply is a piece of cake compared to replacing a laptop screen.