Rondolet- There was another huge thread about the 6600 and FMQ issues just the other day. Check it out:
http://www.quiltingboard.com/t-154486-1.htm

I had the same problem with shredding and some of the Glow in the Dark thread. I switched to a metallic needle ( like I use when I use monofiliment thread) . I got the thread to stop shredding but never was able to get the tension back to where I needed it to be. I've been doing my own FMQ for 8 years so I know a lot of the basic tricks and I still wasted three nights on this tempermental machine. ARGH! I'm going to raise the feed dogs but right now I'm so turned off on quilting with this machine that it's not worth it to sit down and try. Never quilt angry--they say. Good luck.

I have a Janome 6600 too. I always have issues like this with FMQ. if i get the tension right on 90% of the stitches I still have eye-lashing on 10%. The thread (no matter what I use) will shred a few times during the process. The metallic thread needle is a good idea. It does seem to help. If you have any fused pieces in your quilt, my machine will always skip stitches over them. These are just a few reasons I am not sure I would ever buy another Janome machine.

What kind of thread are you using that you are using a needle so large?

How old is the thread?

If you are just using a 50/wt I would try to use a 70 or 80 needle and see if that works. If it's still breaking and you are POSITIVE all is threaded correctly, then it could be the tension.

Hello, welcome from Oklahoma!!!

WElcome!! Have you tried FMQ with the feed dogs up? That seems to be a common solution. (I always FMQ with the feed dogs up and rarely have problems.) I do use a metalic needle since the scaf is larger and seems to protect the thread better. I had the 6500 and now have the 7700 and they both have a tendency to have the thread jump out of the tension disks so if I don't completely rethread I will continue to have issues until I start over, and then cleaning everything -- even one thread in the wrong place can mess things up.

There is a lot of good information in this other thread as listed by Ladyjanedoe.

http://www.quiltingboard.com/t-154486-1.htm

Hello and welcome from northern California!

Welcome from North Carolina.

Hello from Western Mountain Maryland !! :-D

Hello, and welcome from Michigan!

Try a topstitch needle and if you can, change the thread's path of coming off the spool (vertical vs. horizontal).