periodic striped pattern

let me guess he was facing forward or backward based on teh direction of the inconsistency. Based on the appearance and if I am correct about the orientation it was printed in you have a small repeating oscillating shift in the belt controlling the Y movement. A slipping of the belt would be displayed as a stair step that always moves in the same direction this goes back and forth telling me it slips into a new position and then slips back to the original position and this motion sounds like it could be the gear attached to the pulley on the Y motor may note be in the flat spot on the motor shaft or the flat spot is not all that flat so I would check to see if the gear is slipping on the shaft

 

Actually in this case I kind of like the effect