The topstitch or quilting needle, tho my repair guy seems to think a 16 will do anything. Have you checked your needle plate/hole for a burr that might be wearing your thread? And feel around the bobbin/bobbin case also. You can smooth it off with emery cloth. And look for bits of thread in the thread path or tension disks if you can get to that area. Try flossing the thread path with your foot up (unwaxed floss.)