Sorry but I can't help but wonder if the repairman wasn't sure how to do timing. He tried what he thought was a short cut, changing out the bobbin case and charging you for it. He can also get good $$ for the original bobbin case as it is "vintage". I'm not sure he is really reliable. Unless you have worked with him before and know him to be honest and trustworthy, I would take it to Cathy as Bojangles suggested. And yes what Joe said about plastic bobbins holds true. 15-91's were made before the plastic era began to boom. Mine skips like crazy with a plastic bobbin-- ask me how I know!!