nervouse ammeters can be something or nothing. can be a ssimple as a cheap non dampened movement.. or as said with the vreg, brushes.. etc.
one quick test is to full field it. while driving over pasture. if it jiggles at the top end of it's movement.. it's a nervous ammeter.
if it bounces wildly.. it could be a brush or vreg contact issue.
I've seen some pretty JUNKY vregs before.. even new ones. I bought a repalcement for my IH H and the contacts didn't even strike SQUARE...