I use my 6600 to fmq all the time. My tension is usually set at 4. I use essential thread all the time. I had a breaking issue on one quilt and for the life of me can't think of what I did to fix it. I remember doing most of the things Patrice suggested. I believe I got my bobbin case at Allbrands. Sorry I am not much help, but I sure understand your frustration.