Jim,
I am not an expert ( just for the record
).
This is just a thought, but take a closer look at the pressure plate, in particular the 'fingers' that are pushed by the throw-out bearing. If you think about it, the pressure plate is the one moving piece that you have not been able to observe under operation.
If you have the time and ambition ... here is an experiment.
Take the whole flywheel, clutch disc and pressure plate assembly off the motor (be sure to mark how they were on assembled) and set it up in a Arbor press.
- Using a steel disc in place of the throw-out bearing, use the arbor press to gently actuate the clutch.
- Look to see how consistently the pressure plate un-clamps and clamps ... does it move a consistent amount each time? I am not sure how much it is designed to move, probably not very much, so you might have to use some feeler gauges for a more accurate measurement.
Hope this helps ... Best of luck.