Open up the pedal box. You will see the design is a joke - the pots simply
sit in a flimsy way and as the attachment from them to the pedals gets older
and flexible, the pots can simply turn in their seat throwing calibration
out.
Fix - pull the pot out - there is a *** beneath it that is "supposed" to
keep it seated "properly". Cut off about 1cm from the end off an ink pen's
plastic tube and squeeze it over that *** (opening the end of the tube
out a bit with a fork is useful). Slide the pot back in place with the tube
squeazing down into the section below and you now have a rigidly seated pot!
10 min job.
I have done this twice with 2 different units - perfect afterwards.