PXE boot problem

I have a Netware 6.5 SP5 server running Zenworks 7.1 imagaing. Several
workstations are able to perform a PXE boot without any problems. However,
on other workstations (all Dell Dimension 4550), I recieve the follow error
during the PXE boot process:

heap:dispose fail
heap:dispose fail
ERROR: No response from port map service

These workstations are able to recieve the DHCP information fine and have
the correct IP address for the proxy server. I have tried updating the bios
to the latest version. I have also tried plugging in a Intel 10/100 PCI
network card and recieve the same errors using this card vs the embedded
NIC.

The server and all workstations are connected to the same switch. I don't
think it is a switch issue because some workstations on the switch work
fine.

Any help would be appreciated.

Thanks,
David