Ah, I see. It appears that there is some difficulty in reaching GPIB0 in your spy captures. When you next find yourself in this state where it's hanging, try pinging the GPIB-ENET/100 before you reboot or do anything else. My bet is that you won't be able to ping it because your network connection is down. The hang happens because the PC has an established connection to the GPIB-ENET/100, then it goes away. The PC tries for quite a while to get packets through to the ENET/100, but eventually fails, realizes the connection is dead, and gives up.
Keep us posted.
Scott B.
GPIB Software
National Instruments