Sorry, I would have popped in sooner but one of our fur babies passed unexpectedly and I've been a wreck.
The only other thing I would mention is -usually- thread breaks because of a too tight condition: burr, tensioner set too tight, needle too small, etc.
If your tensioner has been moving by itself and loosening, the post that the whole tensioner is on has become a tiny bit cone shaped - with the narrow end toward you. I think Rain has a post about gently widening the post so the tensioner setting stays put.