[Avanti925]

Something really weird happened. after shutting down my G4 533 (System 10.3.5), it immediately restarts. The only way to shut down is to press the power button for 3 or 4 seconds. Then pressing the button will not restart. I must unplug the computer for 3 seconds, and plugging it in automatically restarts it. Could the power button be stuck in a mode? Input is appreciated..

[Big Mac]

It sounds like a power supply problem. You may want to reset the PMU first, before you bring it in to a repair facility.

[Detrius]

Sounds like a stereotypical PMU problem. You need to reset the PMU. Directions are in Apple's Knowledge Base.

It's also possible that the backup battery is dead. You can check that with a voltmeter. It should be 3.6V or higher. 3.59 needs to be replaced.