It could be so many things...could be speed at which you are stitching, I had to learn to slow down. I hope this helps.

I feel your pain. I am going through that right now. It finally did the FMQ and now I am having thread breaking while using the walking foot to sew on the binding. I tried everything above too. Sometimes I guess it just happens. Will wait to see if someone has the magic answer.

i use and love the machine quilting threads from Connecting Threads

I tried practicing on a block of muslin, poly batting and no thread breaking. It is either the cotton batting or the white background material, not sure of it's contents. Confession time, lol. The thread ravels and then breaks if that makes sense. I have tried cotton and poly/cotton thread.

I changed the top tension. It's not helping a whole lot. I don't know how to change bobbin tension.

I don't have all the answers, but I do use Sewer's Aid on the thread to prevent breakage. Works like a charm! It is a liquid silicone. I have been using it sine the early 90's and it has never 'gummed up' my Bernina.

cotton covered cotton thread.
darning foot (with spring on shaft)
fast pedal speed
slower speed on moving quilt with your hands
feed dogs down (usually)

yep, done all that. thanks

Never tried that.