Yep. I've had this a few times now. For me the fix has always been the same..... open the Mac up, remove the battery and press the Motherboard reset once.. sometimes twice after trying it after one press.
It was the P-Ram getting totally screwed up.. not even the old Zapping (Alt, Apple, P and R) solved it.. only this button pressing procedure.
Can't say for sure its the case with yours, but get your Dealers techs to do a Motherboard reset, and maybe a battery change and that should solve it.
It happened to me in a Major, Major stars home studio which was no fun at all I promise you.. thank god for the Motherboard button lol
Note: theres no Motherboard reset button on the Mirror Door G4's.. only the Reset-all command in open firmware...
Hey, Hey!! It's a party... I want to party too... don't ask me about G4's tonight please!!!