XP-Visit internal Server very slow?

Hi All, We have 3 Main servers
(1)DC/windows 2003
(2)FileServer/windows2k
(3) Windows2003. All servers and client-Pcs joined a domain.

Clients open their PC and log on with Domain\UserName and Everything is OK.

But 30 mins later, if click the mapped drive (or shared folder) from server
(2), everything is OK. If click mapped drive from server (3), the computer
take a long time to response. Some time need to logoff again.

For my own PC (XP), I used the domain\administrator to log on, no problem at
all.

I have run the "net config server /autodisconnect:-1" on server(3). The
problem remains.

Please help.