I believe a sensor in the tonearm measures the angle of the cartridge, and sends a signal to the motor to move forward when the cartridge is no longer at 90 degrees to the vinyl. If something (sticky grease, loose or tight belt, dirty sensor, dirty connectors, etc.) does not allow the motor to advance the tonearm and the cartridge gets further away from 90 degrees, does the sensor tell the motor to run a little longer to advance the tonearm and get the cartridge back to 90 degrees?
If so, it's possible that the shorter impulse from the sensor to the motor when the cartridge is only a little off of 90 degrees does not create enough torque in the motor to advance the tonearm.
Japanese engineering is generally excellent, so I do not believe there is not enough torque in the motor multiplied by the pulley diameters and worm / spur drive components not to be able to move the tonarm mechanism buttery smooth.
I believe movement is semi-continuous in discrete steps and basically FINE movement in the horizontal plane and not detectable by eye. The discrete steps are in the divisions of an optical shutter wheel.
The optical shutter wheel is clearly shown in Spikke's picture at the opposite end of the shaft of the belt and pulley. You can clearly see the LED and sensor on either side of the shutter on the pcb. Everything is engineered so that shutter, pulley diameters, worm / spur gear pitch and teeth count make this movement imperceptibly non discrete = buttery smooth. This movement was designed to keep the tracking error at the specified angle.
It is clear that the engineers could not / did not design for old decrepit grease resistance factor or probably taken into account 10yr tune up servicing
Fred and the belt was engineered to "give" for vibration isolation and in case the motor didn't turn off when reaching the limit switches, the belt would be sacrificed and could easily be replaced, avoiding a burned out motor......This system is at the heart of every Technics linear I've had my hands on.....
Music is Everything....Except Predictable....WFUV Fan.