A once a week drop

We are in a Win2003 environment and a year ago added an Xserve (10.3.9) for the graphics dept. Four weeks ago, each Tuesday at around 9:20 am the users get logged off and cannot reconnect. The IS dept comes over (they do not know anything about our Xserve), and the solution each time has been to unbind the Xserve from the network and then bind it back. The users reboot, and all is well until next Tuesday. Does anyone have any idea what could possibly be taking place? No one is doing anything different at that time, at least not on the Mac side.