A couple of suggestions...... first, on your problem machine try clearing the CMOS. This will at least rule out the possibility of a conflict between the BIOS and the hardware information stored in the CMOS. To clear the CMOS, just click here for directions from your model's on-line Service Manual.
Secondly, regarding the Intel Boot Agent, make sure the boot order doesn't list Legacy LAN or LAN as a boot device. Also in the Integrated Peripheral sub-menu, make sure the option for OnBoard LAN Boot ROM is disabled.