System wide keyboard commands not responding

Question

My mac stops responding to system wide keyboard commands such as the Dashboard, Expose and Spotlight shortcuts. It also does not respond to Command-Tab to switch between applications. Hitting the keys for any of these leads to no response whatsoever.
I have run into this problem several times over the last few months, and the only solution I have found is to restart the computer. Initially, it appeared to be related to the ScreenSaver process crashing. I could look in the System Monitor and see that this process had gone red, when I forced the process to quit, the keyboard commands were restored. More recently, though, this has not been the case, and I have had to restart.
Any guesses what could cause this or suggestions on how to diagnose it? Thanks for any help.

Answer 1

I'll answer in reverse.
When I've experienced the problem, I have tried other (older style) keyboards, and that did not resolve the problem.
I've never tried creating another user account.
I'll try disabling the screen saver, but part of the problem is that this issue is sufficiently intermittent that it take a while to determine if this has a benefit. I was hoping to find some more systematic way to diagnose what has been happening. Any suggestions along these lines?
Thanks for your help and interest.