You may want to check to see if you can disable the network boot PROM. That's most likely the problem, that the Netboot sequence is stopped. Some Network cards have boot PROMs built on to them that take over the boot process as well.
Check your BIOS, and see if "Network" is an option in the boot sequence. If so, remove it, and/or check to see if there's an onboard boot PROM that you can disable.