Something quick to check on the US2 is to make sure that the metal connector from the US2 cable is screwed on tight to the Minn Kota adapter cable that runs to the 788c HD unit.

Other than that the best way to check to see where the problem is to either hook up the stock transducer temporarily to see if it still happens or try testing your 788c HD unit on someone else’s boat that is running a Humminbird (should work with any 200/83kHz DualBeam or Side Imaging model).