Is your backing/quilt sandwich hanging near the needle? Check out the needle plate screws.
I repaired a machine today that had been stitching poorly and the quilter felt the machine catching on something when moved left-to-right. The machine was slightly out of time, but after I re-timed it as I moved it across the quilt sandwich you could feel something catching/dragging in the needle area. I got out the handy borescope magnifying camera, scanned the needle plate looking for a burr perhaps caused by an impact due to whatever got the machine out of time ,and found the culprit - one of the needle plate screws had a very significant burr on it.
In the screws tightened position, the burr was oriented to catch on the backing more easily when the carriage was moving left-to-right. I had spare screws, installed them, no more fabric drag, and a happy quilter. It is possible that the timing "incident" could have been caused by the sandwich hanging long enough to cause enough needle deflection to knock the hook out of time. If I did not have a spare screw, a few minutes with a nail file or sandpaper would have easily removed the burr on the screw.
Quilt on!
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