[Old Barney]

How about the transducer itself? I had the unit you are using and had problems after a few years in a totally salt water enviroment. "Junk" and barnacles grow on the flat "reading" surface and all kinds of crazy things start to happen on the screen as signals are blocked (or partially blocked).

I sold the unit with the boat last year and found the trick to be using transducer paint after cleaning the transducer face each spring. This retarded the growth (like bottom paint) and it worked OK.
I think Lowrance has some info on their site about cleaning the transducer face safely.

As Fridays Off pointed out, I think the idea of factory settings reset is a good one as well. One thing you did not mention was any recent addition of electrical epuipment, etc. that could interfere with the signal. Good luck-John