Well, I found the problem.
In my case, it was because my entire drive directory was hosed. I had to reformat it and reinstall everything -- the OS, all apps, everything. Fortunately, it doesn't look like I lost any critical data. And now the machine runs better generally.
So if you're having this problem, I'd recommend you (a) BACKUP, then (b) run Disk Utility, then (c) run Tech Tool if Disk Utility finds problems it can't fix, then (d) wipe and reinstall if the problem's not solved.
Good luck!
--Tom