I often have an issue where my iMac won't shut down, restart or logout users. Basically, when I click Shut Down or Restart, the desktop icons disappear as if the computer's getting ready to shut down (the desktop wallpaper remains) and it just sits there and won't shut down. I have to hold down the power button on the back of the computer in order to get the computer to shut off. This often happens when I try to logout a user. I'd say this happens about 2/3 to 3/4 of my attempts to shut down or logout a user. Any ideas?