Yep. It sure sounds like an encoder/rail issue. The best I have found is scraps of batting on all rail and wheel surfaces. You might be surprised the amount of dust, etc. that will show up on the batting. It only takes a bit to interrupt the signal between all the points that regulate those stitches. I try to do mine between every quilt and after the machine has set unused for a while, even if covered. Sure does make a difference.