Sorry to hear you are having problems with the 6600. I have had my 6600 for six years and it fmg beautifully. I use every type of thread possible. I use a lightweight 60 or 70 poly for my bobbin thread and it is fantastic. I have to tell you that I had the blue dot bobbin case and tried it just twice. IT IS AWFUL!!! I now NEVER use any bobbin case but the original. I always use a topstitch needle, mostly size 80/12 unless I am sewing very heavy fabrics on my quilt. The 6600 is an absolute dream and is my work horse machine. So try not using the blue dot bobbin case and see what a difference it makes. I always adjust my top tension for every project on scraps of the project. Good luck to you.