A couple of things come to mind. Did you try a different bobbin or a different spool of thread. My Grand Quilter sometimes is finicky with the thread (even when it's a brand I've been using all along. Sometimes there is a spool that's just "off" and the machine doesn't like it.) When mine starts breaking thread it's usually tension-related like Lori said. Try "flossing" the tension disks with an index card or a thick piece of thread and see if you can get any lint out of them that way. You might not even see any lint but it could still help. Good luck.