W2k server and domain connections

We have about 20 nodes accessing our LAN. Users intermittently lose their full domain connection preventing them from accessing directories on our Primary domain network drives. They have already successfully logged onto the domain and ultimately notice they have no access to network drives at various times throughout the day. In addition, they are unable to logon to the domain as themselves when they reboot. They receive “No domain server is avail. to validate your connection. Furthermore, they can logon to the LAN under another username until their account resets itself. Why are they losing their network connections in the first place?