I spoke with Humminbird tech support today and learned that because my unit has been retired, and because it's close to 6 years old, it can no longer be serviced
They don't stock the parts for it any longer.
I was told that it's probably a defective screen that's causing the problem, but they're no longer able to accept any of these units for service due to their age and the lack of available parts to repair them.
I guess a new unit is in order for the winter months. Time to do some homework