Bobbin trouble on Singer 403
I have a Singer 403 machine and the bobbin occasionally locks up and jams thread down in hole. It sews along just fine and then will jam up. My repairman looked it over and said he could not get it to jam. It happens when I am starting and stopping frequently while sewing. Hope I am making sense here. Any suggestions on what can be the problem? I am using a bobbin 66.