login problem

I have a small lab of computers in an AD environment and for the last couple of days users are not able to log in unless i unbind and rebind them to the domain. Sometimes this doesn't fix it right away, and even if it binds successfully, if i use dscl to browse into AD it will return "cd: Invalid Path". Once I get them all working again, they are fine until the next morning. Most of the machines have the system log error 'could not determine closest domain controller' and i have specified the preferred domain controller in the plugin (fully qualified) which doesn't seem to help. DNS is working fine. I upgraded to 10.4.11 but that was at least 2 weeks ago and the problem is only a few days old. I'm puzzled because i have not made any changes recently and the problem is specific to the lab but not to other macs in the building. They are on a subdomain, but pcs in the same domain have not had any problems. Anyone have any ideas to help narrow this down? thanks for reading.