I have the same problem on a gigabit ethernet sawtooth g4.
I shut it down, it shuts down, then restarts. Every time.
I have:
- run apple hardware test (all fine)
- booted from four different systems (OS 9.something, OS X 10.3.9, 10.4.3, 10.4.9)
- off two seperate HDs (disk one had: 9, 10.3.9, 10.4.3 disk two had: 10.4.9)
- set all energy saver prefs to never/not selected (in all systems)
- set energy saver scheduled shutdowns (10.3.9 only... didn't help)
- disconnected ALL peripherals (even the 3rd party VGA monitor) except the original Apple keyboard (working blind: hit ctrl-eject, wait, hit return)
- tried two different Apple keyboards
- reset PRAM (five chimes)
- reset SMU/PMU (button above the battery, unplugged, one press only)
- flogged a dead horse
and the problem persits. Every time. Driving me mental!
What can I do???
(only metaphorical animals were injured in the making of this post)