I feel so fortunate that I found this thread! I just took my 6600 in to the dealer about this exact problem! Loops on the back and I had followed my manual. The guy said the upper tension needs to be set much higher than what it says in the book! He set it to 9. Now WHY wouldn't the manual say that?!!! I had cleaned out my bobbin case, so it was clean, but he used air to squirt into the area where the top thread runs thru the tension disk above the needle. He said there was thread fuzz in there (I had just gotten it cleaned a few months ago actually) but was using "old random thread spools" for my fmq practice the Leah Day way. After he did these two things it worked well for him. I got it home and started only using a better quality thread, with new needle and it worked fine for awhile, but even now, at random times, my 6600 Janome gets finicky and threads up in massive knots underneath my fabric (drives me NUTS!). So I rethread and start over again. I am about to start back fmq on my old $99.00 Wal-Mart Brother sewing machine! Janome needs to address this issue I think.