I had the same thing on my Lowrance 522. When ever the trolling motor was activated the screen would have vertical bars of gibberish scrolling across the screen and as soon as I let up it resumed showing good returns on the screen.

Now do to my fast approaching Alzheimer's condition or a couple years passing I can't recall exactly what I done to correct it. It was corrected by changing one of the settings from automatic to manual or vice versa. Probably sensitivity but it worked and isn't a problem anymore.

Good luck.