Have you added any "new" email addresses to that address book? it would be interesting to see if they also went missing.

Windows has a shadow copy of old version of files and trots them out in the background if the file is corrupt on disk

I haven't had any email addresses go missing, including this one. It just reverts to an old name associated with the email address. I keep changing the name, but it doesn't stick. There must be another database that it's drawing from other than the Address Book. I can't find it.

That wasn't it either. I came in this morning, checked my address book under All Address Books, and the name I entered yesterday was gone and the old name was back. The Collected Addresses did not show either the old name or the new name.

When I receive an email from this source, it has a name other than the one I put into the address book. So I edited it from within the email by clicking on the name. We'll see if it stick this time. It did show that it was hiding in my Personal Address Book, but I didn't see it there before when I looked.