I had that very thing happen when I changed an old freewheel with a matching new one. I wound getting not only a new chain, but also chainrings. The old ones were really worn down. Everything worked fine after that. New Shimano rings at bikewagon.com were cheap. If I remember correctly, the 53t ring was 19.95 and the 42t was 9.95. I think I saw them on bikewagon recently both at same price, 9.95.