I know this is an old thread but it popped up when I did a search. I looked at Leah Day's site and she recommends using Isacord 40 wt polyester in the top and also bobbin. She has a great explanation of how polyester threads today are different than the older ones. I tried it and it has been smooth sailing all afternoon. Beautiful stitch top and bottom. I am using a microtex 90 needle.