I have a 64 Mercury, 390, that the ammeter needle jumps constantly. With the headlights on, it jumps so badly that the lights flicker. I was told that putting a 50 ohm resistor in the field line of the regulator will cure this. Does this sound like a good idea?

 

I don't know if the resistor will help, but it does sound like the voltage regulator contacts that control the field current to the alt/gen are bouncing. New regulators don't have this problem and if possible replace the regulator. The field coli of the relay may be responding to voltage fluctuations in the system which could be caused by a questionable connection in the system. Make sure all grounds are clean especially where the regulator is mounted.