I had an interesting problem with my sewing machine. The stitches were wobbley!!! I'd never had this problem before. I threaded and rethreaded and still had the problem. If I hadn't just had it in the shop, I might've thought I needed to have it cleaned. Then I noticed something!!! The BOBBIN was wobbling in the casement! I took it out and compared it to other bobbins and it was about 1/16th smaller!!!!! I tossed it, put in a new bobbin and VOILA -- all is okay!
Thought this might help you.