I used these quite a few years ago with a SLC/504. I remember them being very spotty. They would drop off the network just like you mentioned. I remember we rigged up the PLC to handshake with FactorySQL, and if the handshaking stopped, the PLC would drop a relay that was powering the Net-ENI, which would restart it so the comm would come back up. Yuck!
There is a much better alternative though… I forget what its called, the PLC guys around the office will know, I’ll post back tomorrow morning.