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...