Mail not displaying certain messages

Mail is suddenly not displaying certain messages in mailboxes. I can see and select the message in the center pane, but when I click on it to display, I get the "Loading" text and the spinner. This continues without resolution, though sometimes the messages will display after quitting Mail and/or rebuilding the Mailbox.

It is not random messages that do this, but always the same ones that refuse to display. But they will display sometimes after a rebuild, so I know they aren't gone.

Some have image attachtments, others do not.

This just started this morning, following running Disk Utility to repair some HD corruption last night. Prior to that, Mail (and Safari) had been running very slowly for about three weeks.

I have already repaired permissions on this drive.