Can't stay connected to Wired LAN

We have a refurbished Latitude E6400 Laptop with docking station (running Windows XP).  The laptop works wonderful, except for one problem.  We continuously get kicked off of the wired LAN and we can not reconnect without rebooting the laptop.  So far we have tried updating the drivers, updating software, plugging the LAN directly into the laptop rather than the docking station, changing out to new cables and so far nothing has worked.  We are connected for a few minutes or maybe an hour you just never know how long you have, then we get a pop up asking to re-logon the network, it keeps popping up even if you try to re-logon until you reboot the entire system.

The desktop that was previously used at this location never had connectivity issues so I don't believe it is on the network end of the spectrum.

If anyone has any ideas we would be more than happy to hear them.