So, an update, in case anyone else is having similar issues. I ran out of patience with the problem, so I used the backup/restore utility to create a backup, reflashed Diablo, then restored from my backup.

And then the problem came back. Hair-pullingly frustrating. So my latest step has been to reflash and NOT restore from backup (I just manually redid my RSS feeds, will manually recreate bookmarks, etc.). I'm hoping this will fix it. If it doesn't, I'll be forced to conclude that I have a hardware issue of some sort.

If anyone has any thoughts, I'd be curious to hear them.