My Brother 1500 had the same problem. The machine has a three hole thread guide bar. The manual shows threading under and over the holes. I use one hole as a guide, thread going down not under and over and now the needle stays threaded. Something is keeping tension on the thread before the tension disc for the needle to come unthreaded. Find out which guide is causing the problem and by- pass it and see it that helps. The Brother dealer and repairman wasn't any help at all. The repairman wanted me to buy a new thread cutter.