I had similar issues with my trolling motor and sonar. I tried everything mentioned here and then added ferrite core filters on both positive and ground on both units and that solved all issues. The wires don't just pass thru the filters but pass thru and then thru the filter again causing a loop in the wire thru the filter. This was a last effort after everything else failed but it worked. Even made my am radio work properly. I suppose having a well grounded boat is comforting . My problem was specific to only the upper frequencies associated with the down view functions of sonar.