I have a powerbook G4, used daily by students in a television class.
The laptop, when starting, shows us a screen that says "You need to restart your computer. Hold down the power button for several seconds or press the restart button". It shows the message in several languages.
I started the computer with the install disk, went to disk utilities, and tried repairing permissions and repairing the disk. It said everything was okay. Restart-same problem.
I did a complete reinstall of the system software. When it restarted after the first disk, same problem.
I then restarted in the safe mode, using the shift key. It then asked for the second install disk. After the installation was finished, I tried restarting the computer. Same problem.
It could be something like a faulty logic board. Or maybe not, but whatever it is I'm betting it will need to be serviced by someone. That usually indicates that a part of the hardware needs to be repaired or replaced and for laptops I always suggest finding proffesional help to open them up.