Periodic browsing and logon issues

Network is running W2k AD Mixed Mode. Users at one site will be able to browse the network and login without any problems, and then both will stop work for a while. When they are not working, searching for a computer still works. Domain has one W2K server at one site and another W2K server at another site connected via a 56K PTP connection. The second site doesn’t have this problem at all.

Any ideas on why this is happening? We’re stumped.