Had the same thing happen to me. I put in new brushes (the old ones did not look too bad) but the problem persisted (and was intermittent which was frustrating).

I considered the swapping the generator with an alternator, but decided that I wanted to keep things stock. Since I had the genny out I got it completely rebuilt at a reputable auto electric place (about $100). Although my car has only 87,000 mi the generator needed quite a few fixes but it works like a charm and the amp light never glows even with the all lights and fan on (I don't have a stereo, only the original radio).

I know many swap out the genny for the alternator, but things work fine and look stock with a good generator.

waggin

1965 220 wagon
1985 245 turbo wagon