There is tension difference when the presserfoot is up vs. down so take that into account. Is it every spool of thread or just one? If it's just one, I would test the tread for rot. (Even "new" spools sometimes have problems.) Unwind about 12" or so and holding both ends, give it a good firm yank. If it breaks, it's your thread. That's always my first check since it's easy to diagnose.