oregongirl,
If the machine stalls and then turns backwards it will break the thread.
The best cure for that is to make sure the treadle pivot points are clean of old oil, grease, thread, and dirt, then oil them good so it turns easily and do your best to treadle in a smooth rhythmic fashion. I had to turn the hand wheel by hand continuously as my foot was on the plate for quite a bit until it clicked in my brain and foot what I needed to do.
If the thread tension top and bottom is good, the needle is good, you've eliminated dirt and lint and fuzz under the needle plate, and you've got good thread, I'd be thinking the machine is stalling and trying to turn backwards on you.
Joe