Reason it happens is when engine is subject to idle control throttle pot below 9% and idle switch engaged. ECU controls idle speed to 750 + or - 50 RPM. So if engine is idling at 750 and load is applied to it, the CPS detected RPM will reduce, so ECU causes quantity servo to give more fuel to maintain 750 RPM. When load is reduced and CPS detects a change in RPM the ECU will cause quantity servo to reduce fuel to maintain 750 RPM. So basically only a very small portion of the position or feedback track is being used, therefore it wears exponentially to the rest of the track causing loss of contact in that small area.