I have an analog Chrosziel Mag Fox with a Heden 28VP. Last year I had the potentiometer replaced as it started to kind of vibrate. Everything was fine then but on the last shoot I discovered that the motor gets very slow at calibration in one direction and even stopped turning before the end sometimes!! So my AC helped turning the motor by hand and it worked out. (by the way, it wasnt really a stiff lens!) That problem only occurs during calibration, never happend during operation yet.
Maybe some of you have discovered that or know what it could be. Appreciate your help! :-)