First; I would check to make sure everything is in proper alignment. Correct if needed.
Second; I would remove the bobbin assembly and clean it thoroughly. Reassemble it correctly making sure the wheel shaft is properly lubed and spins freely and the winder will stay in place against the belt without you holding it. If I remember right there is a very thin thrust washer between the winder and machine body, so make sure that's there.
Third; I'd take some alcohol on a rag and clean the belt. Make sure it has no oil on it's surface.
Forth; check the belt tension.
That's really about all I can think of to do.
Joe