I also have a Janome 6600. Mine does the same thing from time to time. I use cones of thread and also spools. It doesn't seem to matter. Clean the bobbin case and check for little pieces of thread. Try switching to a different brand of thread? There seems to be no rhyme or reason why my machine does it. I emailed Janome that is what they told me. If you look on the internet Janomes do this sometimes.