Problem Restarting and Shutting Down

Often, when I try to restart or shut down my PowerBook I run into the following problem. After clicking "Restart" or "Shut Down" in the dialog box, it briefly goes to that blueish screen as it should, but instead of going through the process of shutting down or restarting (screen turns to black), the waiting or processing icon (the grey circle that spins) appears in the blueish screen, and then the computer will neither shut down nor restart; it simply keeps this waiting icon there and stays where it is. It does not appear frozen as the icon is going in its circle. At this point I must hold down the power button to get the computer to turn off, and then I can turn it back on as I normally would.

I have never had a single problem with my computer, it is always up-to-date, and I take excellent care of it. I cannot seem to figure out a solution to this problem.

I hope someone might have an answer for me. Thanks so much.

PowerBook G4 17-inch 1.67 Ghz Mac OS X (10.4.9)