Clicking menu - strange behavior

I have a Mac Mini (OSX 10.5.2) that I use as a file and backup server, so I don't physically work on it often. I've had the following problem come up in the past and it mysteriously goes away, eventually. Now it's come up again, and I'd like to get to the bottom of this.

When I click on the Apple menu, as I move the cursor down, the choices are not highlighted as I pass over each one. If a choice has a submenu, I can't just click on it and then move into the subchoices. Instead I have to click on a choice and keep the mouse button pressed and I move to a subchoice - then I let go and that choice is invoked. My other macs let me click on the Apple menu (then release the mouse button) and just hovering over a choice containing a submenu opens that submenu). The rest of the menus on the Mini behave the same way. As I said, in the past when this happened, it eventually want away. I tried rebooting the Mini, but that didn't help.

Also, I have the dock set to show and hide. When hidden, if the mouse bumps the screen edge, the dock doesn't come out (like it does on my other macs), instead I have to click to open the dock.

Does anyone know why this happens and what I can do to get the mouse-cursor behavior back to normal? Thanks.