(very) odd problem accessing workgroup resources from a domain

OK, so I'm having a strange issue. I've created a new domain for some
office computers (SBS 2003 and XP Pro) and moved them all into it.
There are some resources that cannot be moved into the domain though.
There's a printer and another Windows 2003 file server that needs to
be kept in the workgroup for unrelated business reasons.

When the client computers first login they can access the printer and
the file shares fine. Then, after oh say 45 minutes or so they lose
access to them. All the same usernames/passwords exist in both the
domain and the file server. The shares and NTFS security are wide
open.

I thought it might be a problem contacting the workgroups master
browser, and it may be so. When I do a <browstat gm 1 workgroup> I get
an access denied message.

What I don't get is the loss of access after some time goes by.

Any ideas?