Take the cover off the regulator, and see if the cutout relay points (the ones behind the BAT wire) are cycling on and off rapidly. That can cause the ammeter to fluctuate and sometimes it can be cured by raising the "closing voltage" by tightening the spring for those points a TINY bit. A 12V regulator should have a closing voltage of 12.75V or so.

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Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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