The blur could be backlash or scanning interval.
Have a look at this thread to see if the blur looks like this.
This video from Russ Sadler talks about the acceleration and deceleration of the head caused by the belt drive. I’m curious if the wobble in the drawing coincides with the tooth interval distance on the chuck motor drive pulley multiplied by the ratio of the pulley diameters.
Reversing the belt as Russ’ solution:
I’m thinking it’s this because of the shape of the wobble.
See how the down-arrow absences land between the up-arrow absences.
I think it’s speeding up and slowing down.