Inspect the speed controller. Sometime the nichrome wires break or the finger contacts wear thin and don't engage properly. If that is the case, get a new, solid state type for your existing wires, if they are good. You can get a solid state controller from Sew-Classic.com. I am replace all the vintage controllers this way. You will never want to go back. Oh yes, some of the old ones get full or dirt from shoes over the years. While you have it open, dump it.