On an old auto 6 volt (generator/regulator) system, what causes the ammeter to fluctuate between zero and -35 amps? Seems acceptable at idle but fluctuates on the discharge side when driving. Will occasionally zero out for a second, then begin fluctuating again.
It says on this write-up, that:
... at higher engine speeds the output of the generator will actually drop off quite a bit, as the brushes are lifted off of the armature by centrifugal force.
In this write-up they were suggesting an engine speed of above 3500rpm. If this is occurring at engine speeds lower than that, it could be that the springs which hold the brushes are worn out and lifting prematurely. You may just need to replace the springs/brushes and give your generator a tune-up.