if the thread is shredding- maybe the needle has a 'burr'? I would change needles, completely re-thread from spool- and try again- if it is still breaking- time to try a different thread- even when you use a thread you've used hundreds of times- sometimes plain & simple- it's the thread this time...I've caused myself all kinds of stress/frustration-spent hours trouble shooting- then finally switched thread & all is well. just one of those things- when all else fails- try different thread.