I had several G4's die on me with the same fault.
I went through a procedure of stripping out any the PCi card ( I had some USB 2 cards and firewire cards fitted ) and the AGP video card.
I then reset the cuda button. Took out and put back in, or replaced the pram battery.
Put it all back together ~ held in the power switch and they booted up OK
I don't know which of the procedures restored the mac and it worked on three G4 computers.
I think they suffer from connector problems not making contact and just need some to fiddle about a bit.
Sorry it's not a more scientific answer but as I said it worked on 3 computers that were exhibiting the exact same fault you described.
One of mine has done that as well so I don't know.
It was the G4 I had removed the proccesor from and and then put it back that this has happend to.
The other two x G4 s I tired it with are still working, and of course I still have two G4s that did not have the problem.
I will have another look at mine latter.