I've been having a few weird issues with netbooting with a PI and finally traced it down to the memory card. I was using a micro-SD in a converter, I re-imaged using a normal full-sized SD and the problems went away, used the micro-sd on my PI-3 (without an adapter and surprise, suprise - it works fine).
FYI - You can also use netboot to force upgrade a 2.x net-dimm to 4.02 by following the process below, mine was 2.13:-
For reference if anyone else needs to do this....
Download transfergame.exe and Naomi bios 4.01 and 4.02.
01) You need firmware 2.x on your net-dimm (or non-net dimm if you swap the top PCB over) or you can't get into the network menu.
02) You need > D bios in your Naomi-1.
03) put a null pic into the pic socket on the dimm.
04) Go into setup menu and configure the network (I set mine to 192.168.0.100 as my router uses that range).
05) Plug the network cable from the Naomi dimm into your router, switch it all on and check you can ping it from your PC.
06) in a command prompt (as administrator) run transfergame 401.bin 192.168.0.100 -f -nl
07) your Naomi should display the message screen for updating from 2.xx to 4.01 if not repeat step 06 a couple of times.
0

press the "test" button to upgrade from 2.xx to 4.01, once complete power-cycle the Naomi.
09) Go into setup menu and check the Dimm firmware version - it should now be 4.01.
10) in a command prompt (as administrator) run transfergame 402.bin 192.168.0.100 -f -nl
11) your Naomi should display the message screen for updating from 4.01 to 4.02 if not repeat step 10 a couple of times.
12) press the "test" button to upgrade from 4.01 to 4.02, once complete power-cycle the Naomi.
13) Go into setup menu and check the Dimm firmware version - it should now be 4.02
14) Go into setup menu and configure the network to 192.168.1.2 and you're set for PI netbooting.
Notes:-
You need to set the three jumpers inside the dimm for network & use a known working zero pic.
If you plug the network cable into your router its a normal one not a cross-over as you use with a PI.
I tried using the single file PI netboot image to do this but it wouldn't work so I'm pretty sure you have to use transfergame.exe.
I disconnected the battery so the dimm forgets you've programmed it with the bios when you power-cycle it.
You cannot use this procedure to upgrade to 3.17 (why would you?) as the upgrade process doesn't work and you cannot go directly to 4.02 -
YOU MUST upgrade to 4.01 first.
If you have a non-net dimm with firmware 2.x and access to a net-dimm, take both apart and remove the screw that fastens the top pcb in place (it's near the pic socket) - remove the top PCB and swap them over - then follow the same process. Once you've finished swap them back and you have a non-net dimm that'll boot from compact flash cards.
This process will not work for Dimm firmware version 1.x - that has to be done by swapping the bios chip over.