Troubleshooting VNC and local network connectivity issues

I am having an issue with a cluster of Windows XP workstations and connecting to them with VNC. When initiating a session,  the screen on the target workstation loads about halfway down then the session stalls, breaking the connection. Currently, I'm using RealVNC 4.x server and have tried a variety of client viewers with the same result.

This issue is only prevalent when connecting to any machine in a  particular cluster of workstations that share the same Cisco Catalyst 2950 ethernet switch in unmanaged mode (default boot-up settings). Connections to other workstations and servers on the same lan and IP subnet connected through other switches (two other 2950's with assigned IP's and several other unmanaged workgroup switches do no exhibit this problem.

The workstations in question also typically fail to load the logon script from the domain controller on initial startup because the network adapters lag when acquiring a DHCP address or experience delays in connecting to the network on if assigned a static address. I suspect the two issues are  related and have something to do with the network adatpers. The workstations in question were purchased from the same vendor and installed at the same time all within the past year or so, and older machines on the same switch behave properly. Details about the network adapter problem can be found in this former question: http://www.experts-exchange.com/OS/Microsoft_Operating_Systems/Windows/XP/Q_23586914.html.

Any assistance would be appreciated.