Thanks to all the list members who offered suggestions.

 I think the main culprit is the AFM. I checked with a multimeter and it
had a non-linear response.

I took of the cover and now only say the trace where the wiper had worn
a track in the circuit board, but a small piece of loose 'graphite' that
had been rubbed off. I cleaned the circuit board - I also tried moving
the board and bending the wiper arm to place it on a different part
(unworn) of the track. The screws that hold the circuit board down are
*very* hard to remove.

Though I have not taken a long trip to see if this works, I do notice
that drivability seems slightly better.

Thanks for all the suggestions

Todd