I have the 6600 also. I use the blue bobbin and find the tension needs to be set just a little higher. I think I set mine close 6 or 7 most of the time. I also find if I move my fabric faster, I do better. I read that using a top stitch needle works better because of the length of the thread opening and it has a groove that holds the thread and feeds it better. I tried it as well as cleaning my machine real well and putting a drop of oil in that little spot and things are working much better. With any problem, I always re-thread my needle, for some reason it helps. More fun!!!!