Windows 2003 and Samba Authentication

Running a Windows 2003 domain, with several Samba boxes on the network. From time to time, the Samba boxes seem to drop authentication. We can still See them, and access via telnet etc, but browsing from within Windows just generates a login id prompt, and then we can get no further.

We then either have to reboot the DC or use net join to get them back.

I’m new to the role, so any ideas would be greatly appreciated!