I had a similar problem a few months ago when quilting a throw for MIL. After doing all the standard things: rethread, change needles, clean bobbin area, etc. over and over, I finally discovered a nick in the Aurifil thread spool! It was catching the thread, stretching it out and finally breaking. I switched spools (I happened to have another) and HAD THE SAME PROBLEM! Since both were Aurifil, I am now leery about using this brand even though I know it's well-thought of and others swear by it.