When you've tried all the usual stuff (as already suggested by others), don't forget the thing that has helped me the most in the past when all else has failed...de-lint your machine. Take out the bobbin case and clean the area and down inside...mostly has worked for me. I have a MC9000 and I have found I can use almost any kind of thread with it and have no problems. Most of the time it's the needle or lint inside the machine. Good luck.