Thank you for your insight. To use a common term "on further inspection" (Which I probably should have done before I asked), I think I have discovered my problem. The "stranded" wire to one of the generator brushes has become a "strand" wire. It was apparently rubbing on the rotor. It was either shorting at speed, or I suspect that it was carrying the generator load at low speeds, but not at high rpm? At least it looks like an easy fix to keep as a spare.
Thanks again for your information.