I have the Janome 6600 and absolutely LOVE it. Many of my friends have it and the 6500 and love theirs, too. It is a good machine. I have owned Singer, Bernina, Pfaff and Viking and this is my favorite.
That said, even the best companies can put out a lemon. You might have one.
But, I suspect, like others have stated, that it is in the bobbin area or the threading.
Waiting to hear what happens at the shop when you take it in.