Sounds like you have done some good troubleshooting ..... In addition to what Rickie suggested, I'd give Humminbird a call and see what they have to say. I have been seeing a lot of networking glitches reported on the various forums lately. A lot of them seem to be when I-pilot link is one of the variables.

Maybe consider mounting your other HDSI transducer on the trolling motor so you don't have to share the imaging.