Unable to Connect to the Server

Dear Sir/Madam,
I am administering a netwrok that has Win NT4.0 servers and Win95/98 clients. Tha network is a mixure of 10 and 100 mbps connections on UTPs usign Bay Network hubs and switches. The problem is that sometime some of the users cannot connect to the server and they get the message “No domain server was available to validate you password. You may not be able to gain access to some network resources.” and when the UTP cable is unpluged from the NIC card and pluged again the user will be able to connect.

What is cuasing this to happen???

Thank you and hope to hear from you soon.