thanks for the additional information.
If cleaning the encoder strips does not help, and replacing the scan motor does not help either, then there may be resistance on the carriage.
Usually the carriage is trying to go somewhere its not capable of going, or it does not have enough slack in the mechanical operation to complete the motion.
To fix this, you will usually need to try re-calibrating the carriage location. Also check that the carriage is not loose - all bolts should be tight.
Sometimes you can also fix it without re-calibrating by pushing the head back in the lock position on top of the caps - this should allow you to keep printing until you get a permanent repair.
Another possible cause is a bad servo board.
Inconsistent errors may be caused by a warped or badly shaped carriage - over time it can change shape due to environmental conditions, especially if the plotter is left near a heat source or in direct sunlight.
Your next step would be to fit a replacement carriage to see if that eliminates the error.
Ok, let me know your findings!
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Have a good weekend!