I hope the advice you've been given helps fix this issue for you. I use Guterman and Aurifil threads in my 6600 without any trouble. However, it doesn't like King Tut thread for some reason. My older Jamome is nowhere near as picky.

Most often it is the top thread that is causing the problem. When the thread is made it has to be
twisted or it would all unravel. As the thread is coming off the reel to sew it is twisting more as you
go along and that interacts with the bobbin thread and it gets knotted from the twisting. If you
take the reel of thread off the top and turn it over (top to bottom) it will untwist itself.
So, whenever it starts to play up just turn the reel of thread over. Magic happens. It is all in the
way the thread is made because it has to have that twist in it to stay together. If you are using
an industrial machine it happens even faster because it can be sewing at 6000 to 8000 stitches
per minute. It really is Magic. lol Gleniveve

same here. I don't think that is the problem.

best advice yet.

I have a Janome 6500 and have never had a problem with any thread type. I have used it all including Connecting Threads, Coats and Clark, Mettler, Aurafil, Hemingworth, etc. I use polyester, cotton and rayon. I do not have any Gutterman so I can't speak directly to that, but I know other people use it without any problem. I do use the Janome brand bobbins made specifically for my machine. This could be the cause if you are using other types of bobbins.

As a last resort, I'd take it to a different janome dealer. Sometimes the problem is with the tech doing the work and this may be a problem that just needs a different set of eyes. I've been to the point of madness before and took a machine to a different tech and he fixed the problem instantly and free of charge. You never know...

I have the 6600 and the only thread problem I've had is when I forgot to put the disc on top of the spool-sometimes the spool starts to spin-and that really throws the tension off. If the disc doesn't work-another trick is to use the felt pad on the bottom of the spindle which again stops the spool from spinning.

ditto ditto ditto to all the good suggestions already given.

I got my 6600 in September, so just ahead of you ... and have not had any problems of this nature. Nor with my preceding three Janomes! And believe me, I have sewn miles and miles since I got my 6600.

Just ahead of that, I discovered Aurifil thread ... and started to use it. I heard/read all the horror stories about it being a thread diva, so decided to stick with the Aurifil. And it's been clear sailing!

I tpo had LOTS of Gutterman threads, some Mettlers, but it's hard to go back, once I tried the Aurifil as it makes a much nicer seam! Now that I am more than familiar with the 6600 and know there's no problems ... I have been using my Gutterman stash, and so far, no problems! xxxxxfingers crossedxxxxx!! :)

With all that's been said, I think your best tip is to buy that one spool of Aurifil (or whatever your dealer recommends) ... and give it a try. You have mentioned that your dealer has offered that as a solution, yet it appears you have not taken that suggestion. You can prove them right ... or wrong! If right, then the problem is solved ... if wrong, then let the dealer figure out the solution.

Being that it's working just fine when you take it to your dealer ... it would appear there is indeed something different happening. Are they using your thread? Are you doing any of the sewing? As already suggested ... sit and sew along with your teacher/dealer ... and perhaps there's some little things you are missing when you are home and DIY which are causing the problems.

What she said.

I don't use Gutterman thread at all and never have. But I have been able to use two different weights of thread in the bobbin and the top without a problem.

Another possibility is your top thread may be jumping out of the take up lever. Try threading it from left to right instead of right to left.