Hi Peter,
I find it hard to see how the needle dances all over the place when you put your lights on. Do you mean when the motor is running and is stationary on centre stand? I believe smaller movements are due to regulator oscillation (correct me if I'm wrong someone) in which case cleaning up contacts and generally checking wiring would be in order. An electronic regulator may well stop erratic movement too. There is indeed a small damper screw on the back of some older ammeters but you should check your system before twiddling the damper.