Check for high resistance in your field circuit or a bad load resistor for your beacon. (If you have a beacon, and only found one bad in 25 years) Unplug your beacon to see if the problem continues. If jumpy needle continues, I call this the "nervous needle" symptom. It could be any thing in the field circuit causing high resistance including bad connections at the the field post of the alternator, master switch, etc.... Quite often replacing the crimp connections on the wires at the master/alternator switch or alternator fix this and often I have had to replace the master/alternator switch entirely. What is happening is, the voltage regulator is trying to push a small amount of current through the field circuit to excite the alternator field as needed to keep voltage steady. It builds up at the high resistance point then bursts through and suddenly there is an excess of voltage going through the field of the alternator which results in a jump "up" on the ammeter. The sense wire to the regulator from the buss now sees the jump "up" in voltage then decreases the voltage to the field dramatically thus the ammeter dropping to discharge, the regulator now sees a drop in voltage on the buss and the cycle starts all over again. This is all happening many many times a second. See, it's simple !! Hope this helps !!