Originally Posted by Tartan
When I had a thread snapping, I found it was the thread. It was an expensive machine quilting thread but it was making a little lint ball as it came to the needle. I changed threads and it was fine. If you are using a thread with the little slit on the spool side, that can catch and break the thread. If you are pulling your sandwich or the sandwich has drag on it, that can bend the needle enough that it clips the thread on the throat plate. Have you checked the tension and made sure the needle isn't bent? A bent needle can also clip the thread on the throat plate. I hope you can find the problem.
I agree with Tartan on the possibility of the thread getting caught on the spool. I've also had problems sometimes with the thread winding on the spool pin. It is maddening. I hope you figure it out so you can finish your quilt. If it pieces fine, I would guess it's not your machine.