Win 2k machine logging to remote servers

I have a Win 2K Network with 5 DC’s in same domain at 5 different locations that are connected via frame relay 256 K connection. I have noticed that my 2000 Clients are logging on and getting their Logon scripts from Remote servers even though they have a Local server. If I clear the NBT Cache they get it locally but again in few days they go out to different servers. What I want to know is Why this happens and how can I tell these machine to Log on to a specified server ??
Thanks in advance..