Sounds like either it's hardware or a bad Install disc. Setting up on the iMac should test the latter notion.

For making bootable backups: the 'Restore' function of Disk Utility works like a charm. Run it booted off the CD/DVD, of course.

Thanks, I guess I missed the obvious; running it by booting off the CD so the disk doesn't have open files.

At this point I'm considering a full clean format/reinstall and restore only data, no programs or settings. Do you think that's the wise thing to do considering the random issues I'm having? The things I absolutely must have to do my job are MS Office, Eudora, Quickbooks, VNC, Remote Desktop Client, and a browser (I could live with Safari temporarily). I could run with that limited install for a few days and see how it goes.

The CD passes the integrity check, but should I ask for a new one anyway maybe? The guys at the Apple store seem pretty willing to help, they may do that for me if I show my originals.

It's so hard to try to troubleshoot when the machine is also you primary computer...

Sorry of this has been asked (and answered) before, but have you looked at the crash log and/or system.log from the time of the problem? If you copy & paste the relevant bits here maybe someone will have a clue.

I've looked, and nothing goes into any of the logs. The OS doesn't crash, just the user interface. I can SSH to it and do a shutdown that way.

I did a clean reinstall last night, formatted the drive and all. Reloaded my data, no system or user settings, and reloaded only the most critical apps. I'll run like this for a while and see what happens.