Everything's tight. I's just come out of the shop from its 10,000 mm extrusion service.

Dr Luigi may have hit on the cause - room temperature too high. Roxy, what is the room temperature you are working at when you get your jumps? I was sitting in 36C without A/C, and 33% humidity.

My steppers were pretty hot, but there were no unusual clicking sounds. That is to say, the steppers were not clicking, which is usual for my printer.

If my belts were loose, or the steppers were missing steps, how do you explain the exact amount of displacement of the Y axis?

Old Man Emu