My J2 is not able to move through the full range without stalling and skipping unless I help it move through the "heavy" positions.Verified Dip Switch settings and voltage setting.
I got the same problem and it was the connection between the J2 motor and the gearbox, it's very easy to tighten the 2 screws (green arrow) so that the connection is good, maybe you can add it to your mounting pdf ;)