When you say, "a cotton quilting thread by Coats," is it "hand quilting" thread (with a glaze finish)? That stuff should never go on your machine. It's like feeding wire through your machine, and you can't get a good stitch with it. Another thing about Coats and Clarks thread is the spool has cuts in the ends, where you're supposed to tuck your thread ends. If you haven't got it in a perfect world, the thread will catch and jerk as it comes off the spool.