I am sorry you are having problems.. but I have that same machine and whenever it gives me problems its because the thread somehow comes out of the little take up lever. I don't how it comes out - I must bump it or something
If you took it in for servicing and told them about your problem - they would definitely help you out. But then you said the one place is gone so you may not be very close for servicing.
I would check the threading - when a machine isn't threaded correctly it can really cause problems.
I too wonder what batting and fabric you are using. Is you practice sand which the same as your quilt?
I wish I could help more