Team city loses its ability to authenticate against to SVN

I have my CI loop happily building away as changes are committed and everything is cheeky.

At certain points during the day I am getting the following error:

"Could not get project sources"

The detailed log reveals that for some reason TC has stopped passing the login credentials:
' failed with erorr: org.tmatesoft.svn.core.SVNAuthenticationException: svn: Authentication required for '<http://xxx.xxx.com:80>

This is random and happens after several succesful builds on any given day.  If I defer the fetching of source to my MS build task it works everytime.  Obviously I want TC to fetch the sources.

Please help!  We are rolling our TC in a production fashion next (Just got our corporate PO for Enterprise!)

Thanks for your help on the worlds best CI product.
-Jonathan