Another thought. Have you oiled your tension? I never thought to and it wasn't on any of the instructions I had for my machine. In that instance my top thread kept breaking after a knot was sewn between the bobbin and the needle. I had cut the knot out several times and knew something had to be done. Then I remembered that one of the top quilters in the country, wish I could remember which one, said on a blog that she soaked her thread cone in oil before she started quilting. I got to thinking about that and realized my thread might not be sliding through the tension properly. I oiled the tension and the problem was fixed. And the tensioner didn't oil my quilt at all.