When you say my powerbook g4 15"does not restart immediately after shut downs.
it does however restart after waiting 8-10 hours.[/quote].
'''Sounds like warping of the logic board''', and cooling resets the warp. You could ''try to shim the board, or look for solder joints that need reflowing''.
If that's too much work then replace the unit it's long overdue.
Good Luck,
N.