I am having a problem with my the needle up/down function. When I press the button to set the needle down to pick up the bobbin thread, the movement is slow and jerky and the needle will stay down in the quilt - eventhough I have it set for needle up. I happens regularly when I am in handle bar control and has happened a couple times when I used the ProStitcher. What drives this function? I have looked for loose cable, and that did not seem to be the problem. Thanks.