I just finished fmqing 2 queen size quilts on my 6600. I always start with a new needle (I use Organ needles). Feed dogs are down, and my tension is set between 8 and 9. I set the speed control about 3/4, and always use the foot pedal. That way I can really control the speed. I use thread from Connecting Threads in both the top and the bobbin (no special bobbins, washers, or case) and have no problems now. However, I have had lots of practice. As others have said, if you move the fabric too fast, that can cause the thread to snap. Also be sure to keep your quilt as flat as possible around the needle. Don't give up!