I got my "new" old machine the other day and just fired it up last night. I was amazed with it for about 30 minutes, then it started skipping stitches. It was like the bobbin stopped working for a few stitches and the will stitch a few and then stop. I've rethreaded, changed needles, dusted it.... and it worked fine. Well, it started doing it again tonight! What am I doing wrong???