After upgrading server to win2k3 xp pro slow to connect and looses connection

I have experienced this twicw now where If I change a
server but have all the same information like IP, domain
name etc etc or... upgrade an existing server to say
win2k3 from 2k or to 2k from nt4 all my windows XP
Machines seem to be extremely slow to login and also will
lose thier connection if on overnight. I have tried
putting the XP Machines back to a stand alone state and
tried rejoining the domain but the problem still persists.
What can be done???