Re-enter password

Almost every day, and sometimes several times in a row within a few mintues, my Mail application pops up and requires re-entry of my password for an IMAP account (almost always Yahoo but can be others).

My credentials are correct and they always work once entered. Even if I check the box to remember the password it just keeps forgetting and asking me again.

What's up with this? It was annoying but now its making me mad.

I've tried deleting and re-adding my Yahoo account to mail. I've tried deleting my mail preferences file. No dice. Just keep getting these popups.

Anybody had this and know how to fix it?

Thanks for any ideas!

Greg