I also have the 6600 and have found that there is a specific combination of thread, needle and bobbin that will work for me. I use a 16/18 topstitch needle, poly thread both top and bobbin, and the blue bobbin casing. Although I think it is the poly thread more than anything else. I turn up the tension and keep a very close eye on the stitching (both sides) as it changes while I fmq. I keep the speed of the needle up, and the speed of the fabric down. It does take a lot of practice, but those are the best tips I can give you.