I can feel op's pain, you pay top $$ for a product and the frustration begins to set in when the stuff does not work and you are not able to resolve the issue through the proper chain of support.
Many years back I was about to buy a Mercedes Sprinter Van, and I backed out of it as there was a lack of able mechanics that could resolve the simplest of issues.
I suffered similar fate with my duramax diesel, the chevy mechanic at the stealership I bought my truck at was at a complete loss, they actually kept my truck for nearly a month and finally rebuilt the front diffy.
The irony is that my buddy who works at ASE in Detroit correctly diagnosed the problem without ever seeing the truck.
My experience with Humminbird was good, I have called then a couple of times and they are willing to listen and help.
Being persistent and working with them is key but getting someone who has the knowledge or is willing to take it to the next level is what you need to resolve a not so regular issue.