Windows 2003 server-clients can't connect

I replaced 2 servers recently, one was a win2k server and the other NT4, in
different locations. The new Win2003 servers are both DC's. Before
replacing, using the old servers, the network ran normally, except for the
preformance of the old servers.

Now, in both locations, using different hardware for each server, I will
have a day where everything works fine. Users can log on to the DC, shared
printers on the server work, shared drives map properly. The next day,
chaos. Half the users get an invalid logon message, or the drives don't map
on first logon, logging off then on again maps the drives, some programs
won't work using data off the server.

This is intermittant, but will happen 2-3 days a week. The common
denominator seems to be Win2003 server.