Well, I am new to the player piano rebuilding, but on a Recordo player
piano I rebuilt this summer the airmotor would speed up and slow down
because the stack had old valves.  I had replaced all of the pneumatic
cloth and not the valve leathers; a bad mistake, but I took it all
apart and rebuilt all of the valves.

This Recordo piano had a four-bellows pump similar to an Ampico system.
When the piano would play normal music at slower speed it would be fine,
but when a more lively song was played it would speed up and slow down
depending on how many notes were being played.  So if your air motor is
tight and the air motor governor is adjusted right then this is another
place to look.  If the motor lopes then one or more of the pneumatics
are hinge bound.  Just some thoughts.

David Dunn
Morden, Manitoba, Canada