Change thread. Use 90/14 topstitch needle. Make sure your foot lever is down...and ruler foot closer to your fabric. Thread is in the take up lever,can't tell you how often this happens). Sometimes a microtex needle 90/14 is the answer. I took a free motion boot camp class and was told to bring a microtex needle 90/14. My thread shredded, broke a needle, pulling loops to the top of the fabric.....brand new Janome 8900. I took this machine out of the box, with the factory needle in it, don't know the size, set it up for free motion, worked perfectly. Next day we were off to the boot camp. But I did one thing different. I bought some beautiful thread, never, opened. Finally the instructor came over, person who told me to use the Microtex needle, she tried, same shredding. I took the thread off the machine and happened to turn the small cone upside down and inside the tube it said what size needle to use with that thread. Bought the needle at the quilt shop where the boot camp was being held....thank goodness they had one...and away I went. No problems for the rest of the class. Lessoned learned, match your needle to your thread.
BTW I love my Janome 6600. It is a workhorse of a machine. The 8900 is wonderful too. Best wishes for solving this mystery.Oh, and be careful with the shredding, it can lead to the dreaded broken needle that flies into your face. Ask me how I know.