So often these problems have to do with the threading of the machine, needle and correct placement of the bobbin. Even things like threading the needle in the wrong direction can make a problem. So I would suggest a good quality machine piecing thread like Mettler, a new needle for stitching, re-winding a bobbin with the same thread as you have in the needle, and really make sure it is according to the manual. When you go to remove the bobbin, make sure your take up lever is in the full upright position, and the presser foot is up. Be sure the presser foot is up when you are threading the machine as well. When you start to stitch, make sure you are stitching into fabric on the first stitch, as singers (IMO) are notorious for going wonky if you don't sew into the fabric, rather than catching some air on the first stitches. Hope this helps!