I had a PFAFF grand quilter, where is your thread breaking? If it is at the needle, make sure the needle is inserted correctly. Try another needle. Could be the tension is to tight or the quilt is stretched to tight. Is the backing stretched to tight? If the thread is breaking above the tension guide - is there a rough or sharp spot in the thread guides. Are you using the same thread as you used in the other quilt and same bobbin thread. Does you thread pull easy after the machine is threaded?
I know it is so frustration - been there done that - didn't like it. Let me know if you get it figured out.