It's a manufacturing error. After many times taking mine in for repair, they finally called the main manufacturer and found that it was a glitch in this model.(6600) The newer Jamones the error was fixed.
What you need to do is when you are done machine quilting for the day, set the feed dogs back up in regular position BEFORE turning off your machine. Never turn it off with feed dogs down!!!
That was the problem. I've never had a problem since. Hope this will work for you too!!.