Is anything different about the fabric? Some people have problems particularly with the thread counts of batiks. Sometimes a coating like a chintz has given me issues, heavy "paint" tone on tone ones sometimes too. I like the Microtex Sharps for that.
The other thing is to check the rails and your encoders. I've had a little bit of fluff that would roll around and only give problems periodically. You might think if it particularly happens in any direction and look specifically in those area, like if you are ok going back and forth its not as likely to be the rails as it would be the encoders.