This was the big problem: the alternator pivot bolt was barely hand-tight. So it wasn't clamped onto the engine bracket, and wasn't making a good ground contact between the case & block. The lower (front) alternator tab in the right foreground shows signs of powdery rust (from movement) and a few small shiny spots where it was rubbing intermittently. I'll wire-wheel all these mating surfaces down to bare metal, and then install the alternator correctly so it has a solid electrical connection to the block.

Owner caption August 2017 · Photo 24 of 35