My Janome MC11000 does that from time to time. I haven't tested my theory, but I think it may be the foot pedal. It has not had the easiest life. For now though, I just let it go on as it is only for an inch or so and very seldom. Maybe once or twice during a long sewing session. It also never does it during a embroidery session, so I' sticking to the foot pedal theory.