humming bird 343 fish finder

by Rita
(Trout Creek Michigan USA)

This may sound crazy but when we put the boat in reverse, the finder goes nuts, jumps depths from 11 feet to 600, and all in between. Has its own line to battery, seems like the electrical system triggers its behavior. Could it be that a different line does this? We sent it back to dealer last year and they say its good. Works great as long as we dont back up. Transducer is not loose. What is going on?

Rita