Since you said the rebuilt Bosch unit came from a yard, can I assume that the rebuilding was done at a time in the past unknown to you? If this is the case, I think it's worth the effort to pull and inspect the brush/regulator set, and look at the slip rings, too (use a mirror).

I understand that part of the generated electricity is used to add strength to the rotor field, and that field is what the regulator acts upon. Something in that area sounds out of whack. Maybe the brushes are not up to par, or the slip rings are messed up.

Just a thought.

Good Luck,

Bob

:>)