The new Coats and Clark thread doesn't have the slits on the spool as they did years ago (except for the large Wal Mart spools.) Now the spools are made like Isacord spools or others with the smooth slot to tuck the thread into. I don't think it could get into that slot while sewing. I suspect when it jumps, it either has too much twist or else the polyester may have a coating that is clogging up the tension disks. It is really frustrating.