qmail + courier-imap + vpopmail

Okay, I have a really annoying problem which has stumped me. The problem is that after a random amount of time, courier-imapd will no longer authenticate any virtual domain user. A regular shadow file user will work fine and if I restart courier-imap it'll work for a little bit and then quit working. I know the username and passwords are valid but I don't have a clue as to why it is saying login failed. I've upgraded courier and that hasn't solved the problem. Any ideas would be greatly appreciated..

Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
* OK Courier-IMAP ready. Copyright 1998-2002 Double Precision, Inc. See COPYING for distribution information.
a login test@test.com testing

a NO Login failed.