Sounds like you have the opposite problem of most tachs and electronic ignitions (non-points). If so you would undo what has been modified to allow the tach to work which was the addition of a resister in the circuit. Sounds like you need to remove the resister attached to the negative terminal.
According to the instructions when you convert from points to an HEI you add a resister to the line to make the square wave that the tach sees look more like a points ignition. But you usually add the resister to lower the tach reading high or bouncing all over the place. Those are not the symptoms you described.
I would contact Sun to see what it costs to get it working properly. Or you can contact these guys: