E-man,
I found an old write up that spoke of the hertz surface that builds up on the commutator you spoke off. It describes two methods in which to accelerate the hertz surface layer.
1) use pine wood to burnish the commutator.
2) use a very light layer of commutator oil applied by a rag or cloth.
Solidstate,
I looked through some old papers. There was three items I found for overspeeding.
1) load compensation switches
2) contacts such as H
3) motor field deterioration
Please let us know how you are making out...
