I have a new speedometer and also a new speed sensor installed on my 98 Sportster 1800. The speedometer works just fine when bench checked with an external battery, or if the engines of the boat are not running. Once the engines are started and I start moving, the speedometer indicates speed for about 3-5 seconds, then bumps around a bit while heading back toward 0 where it will stay until the engines are restarted. Checked all wires for connection and proper ground. Boat voltage indicates 13.4 volts when engines are running. Can only get the speedometer to work by connecting it to a different battery (not the boats battery). Could there be some type of line noise getting induced into the power going to the speedometer that would cause it to do this? Would a bad charging rectifier(s) cause this type of phenomena?