Is this a new machine or one that has been used a lot? If it has been used a lot, have you cleaned it thoroughly? A small piece of lint caught in the upper tension knob can cause lots of problems, as can lint buildup around the feed dogs. For the upper tension, it's a good idea to get a piece of *unwaxed* dental floss and "floss" between the tension discs (with tension on the lowest setting). That is likely to dislodge any lint. Cleaning the bobbin area depends on your machine. For the feed dogs, usually you have to unscrew the plate in order to see any built-up lint around the feed dogs.