PXE: no response from port map service

PXE boot from a Dell GX150 (latest BIOS) to NW6.5 SP3 and I always get
this error. It is always preceeded by heap: dispose fail (which appears
twice).

If I plug a dell gx280 or D600 into the same lan cable the error doesn't
appear.

Any ideas? Does anyone know exactly what the error means?

Thanks, Doug.