nt4 pdc drives drop off network

I have an NT4 server (pdc) every week or so all of a sudden users cannot see the network drives on the server. I have to reboot the server. There are no event log messages and the server seems to be running fine. We just moves to a new building withall new hubs and routers, however this also occured at the old building with the old swithces and routers. Any ideas?