you've got something on the encoder strip and the IR sensor is missing steps and having to step out further and further to make up for the missed steps. when you restart a print it zeros and will start the process again. get a white piece of paper (business card), a magnifying glass and a flashlight. put the paper behind the strip and slowly go down the strip looking for a dot of ink. the encoder is printed on one side (a laser copy microfilm type of print). so one side is just polyester the other side is a bit delicate and should be lightly cleaned with iso.
i have a test print on one of my hard drives that is tiny 1 pixel lines that print across the platen. with a print like this you can directly locate when the printer first lost a step. the printer should be in 360x360 one pass to make the results easy to see.
occasionally the IR sensor is at fault. but the symptoms are erratic in that case.