Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I advise you to clean, not the transom-mounted transceiver, but rather the connections from it, to the Humminbird device.
Trace the whole system through, the lines that is, from the tranceiver on the transom-mount, to the Humminbird device..
For each connection, undo it, then brush each side of the connector with a clean dry brush, for 20 + seconds each, then wipe down each side of the connector with a clean dry rag cloth until the metal shines, then re-connect firmly, then go on to the next connection and do the same brush-and-wipe for all connectors and connections along the entire system (with the power Off, of course.)
This is a treatment for corrosion. Which happens often and threatens the proper operation of all electronic equipment and connections in a maritime or lake-going or riparian environment.
Let me know how it goes, please. Thanks... I hope you will see improved performance.