Mail keeps asking for Yahoo password

I have configured a Yahoo mail account under Mountain Lion (10.8.2). Running the Connection Doctor in Mail shows green for both the Yahoo IMAP and SMTP accounts. Account was set up using the System Properties Mail, Contact, & Calendars panel.

Problem I'm having is that each time (well 90% of the time anyway) Mail checks for new mail, I get a pop-up telling me "Mail can't connect to the account 'Yahoo'." and asking for my password. The password field contains my password and all I have to do is click OK and everything seems to work--the account shows as online. I am able to send and receive mail through the Yahoo account.

Does anyone have any suggestions on getting Mail work correctly with Yahoo? Having the pop-up occur is driving me crazy.

In addition to driving me crazy, I provide support to a local Mac Users Group and am trying to convience folk to use Mail to access Yahoo instead of Safari. With is problem showing up its a hard sell.