Sorry I haven't jumped in sooner. I would drop out the motor, to remove that as a contributor. Is it hard to turn for the entire stitch cycle? Does it only stiffen up in certain parts of the cycle? Open up the top, bottom, and nose, and turn by hand while observing each area to see if anything obvious jumps out at you. I would not think that stitch pattern selection would have anything to do with it, but just in case try changing up the pattern and then try hand cycling to see if any changes.