Mail problems: cache updating & retrieval

This is with Mail 5.3 under OS 10.7.5

Updating cache, changing flags, etc. seems to be taking a weirdly long amount of time and cycling over and over.

I have been keeping the activity window open to see what is going on. (First noticed it because outgoing mail was taking forever to whoosh out.)

Then in the activity window I saw this gem:

(rats image file of activity window is a pdf and only 25 kb but won't seem to upload here.)

ok, so one of the bars says: "[account name imap] Opening mailbox. . .

Received 16 of 18446744073707458226 headers"

That's right 20 digits or roundly stated 19 quintillion. I don't even have 5000 messages in my inbox.

What's going on?