Windows 2003 DC kicks users off intermittantly, why

I have a windows 2003 domain in mixed mode alongside a win2k server as secondary. My users will be browsing the files on the data share when suddenly, windows returns the following error:

\\servername\share is not accessible. You might not have permission to use this network resource. Contact the administrator of this server to find out if you have access permissions. Access is denied.

This issue only appears on the one server. This occurs to both admins & normal users. I thought maybe it could be too few CAL's however, we have 4x the amount needed since our company has shrunk within the last 2 years. When the error happens, the user can reboot & all is well for anywhere from 4 hours to 4 weeks. You can also sometimes choose "disconnect mapped network drive", choose the server & disconnect. You can then use unc path to reconnect & be prompted for Ad credentials. Using the same username will not work but another username will be fine until it happens again. I have noticed that when the problem is more prevelent (it seems to do this in waves), if you connect to your unc share, do your thing briefly & then close the window that the problem will not show up as often. Also, This only happens to people using XP. The following OS's do not disconnect, Win7, Vista, Win2k, OS/2, Win98, RHEL5, DOS.

This is driving me nuts ! Anyone have an idea what could be causing it ?

some afterthoughts:

this happens to individual users one at a time not en masse.

event logs show nothing. There are no entries for improper authentication/denial.

This happens at varying times day or night.

Server load is not a factor

network card is not flaking out (I don't think). I've implemented a constant ping to both nic's for days at a time & neither has been interupted, ever.

Some more afterthoughts:
The issue appears on all windows xp machines. This will happen eventually (but intermittantly) on machines using Pre-SP1,SP2 or SP3 with a base install.

The issue does not follow a specific user. A user can use a win2k machine & all is good.

The problem can appear to machines joined to a domain or in workgroup mode.

The issue happens when connecting to shares either through unc, mapped (either in explorer or net use cmd) or browsing network neighborhood.

I have not had the problem reported to me under vista or win7 but since we only have 2 vista machines & 1 win7 (my hack box) then the problem may just be going unnoticed.

Another example of one of the complaints is that a person can no longer print. I go to their desk & am able to verify that it is the same issue of being kicked off & not seeing shares. reboot & all is well (or disconnect share/server,repair,etc…).