XP connecting to Windows 2000 Server

We have a windows 2003 domain within a WAN and have 4 Windows 2000 servers in different locations. Within these locations are Windows 2000 clients. We have recently installed XP clients and find that the XP Clients keep losing connectivity to the windows 2000 server which is located in their on LAN but connect fine to the other servers across the router. All the 2000 machines do not lose connectivity so its not a network problem. any ideas as this is stopping us moving our clients to XP