There's a chance that one of the brush wires are broken & not making a good contact. Or it could be one or both brushes are pretty well worn down. I don't know how long since the Genny was last repaired, but if it hasn't for a few years I would take it out. (1) Give it a good clean. (2) Get new brushes. (3) If the Commutator is out of round or slightly concave get it turned. (4) Get the Commutator grooves (Mica) cleaned 1/32" below the copper & then with #00 or #000 sandpaper remove any rough edges. (5) Check the bearings for slop & change them if required. (6) If the rest of it is O.K. it should last you for the next ten years or around 20,000 miles. Or if you're not a Purist now is the time to change over to an Alternator.
Chris....From the Land of OZ.