If the thread is breaking and shredding, consider the possibility there may be a burr somewhere...it could be the bobbin case which holds the bobbin or the needle plate. The burr on the case can be gently smoothed out with a special stone or very fine sandpaper. If you see marks on the needle plate, then the machine could be out of alignment and needs to be adjusted professionally. I had the same thing happening to me recently and it had been serviced by the factory so the alignment was not the cause. Finally found the culprit....me! The needle was not quite in all the way...just enough to stay in but still slightly down. Once I reinserted the needle, the problem was solved.