I have a Juki on a Superquilter frame. When I have thread breakage it's almost always the needle. I now use a size 14 topstitching needle and seldom have trouble. Sometimes I'll start to have thread breakage after I've been quilting for a while, and it's because the needle is getting dull. Switching to a new one usually eliminates the problem.
My tension problems are often caused by a buildup of lint in the bottom of the machine. Take it off the frame and make sure there's nothing in there. I also take the needle plate off and clean out in there. Good luck!