Have you checked the wick that oils the shuttle? Use a q-tip to soak up a bit of the oil in it and see if the oil is old and black. It might be getting into the shuttle and onto the thread? Use a Q-tip with a little clean machine oil on it and clean the inside of the shuttle also, there might be some dirt inside. If you have oiled the top shaft, make sure to dry it with a soft rag also in case dirty oil is dropping down.