POP downloads don't run on their own. You need to tell it to start
downloading. Either that, or have your email program configured to
periodically check for mail. Clicking on the Send/Receive button in
O.E. does exactly the same thing. It shouldn't start all over from
the beginning, unless you tell Gmail to do that.
While it's true that POP downloads only some of the messages (if there
are a lot), you shouldn't see an "unknown error" message. Since you
do, something must be wrong. How is the disk space on your computer?
In any event, I do think IMAP would do this better than POP.
What happens if you go into Gmail and look for messages starting at
Dec. 13, 2007? I'm wondering if there are some messages on the server
that are corrupted or inaccessible for some reason. Yes, it shouldn't
happen, but like they say, 'stuff' happens.
(I had that sort of thing once in a while from my old ISP. When it
happened, my daily email downloads would repeatedly fail at the same
point. I got around it by clicking the "Stop" button a little before
the point where it aborted, then logging into the ISP's webmail page
and finding the bad message and deleting it.)
Andy