Originally Posted by
JohnDThompson
It's likely a mismatch between new chain and worn sprocket. If the wheel is original to the bike, it will have a freewheel, not a modern freehub/cassette, and probably only five sprockets. Individual sprockets will be hard to source and not worth the effort. Such freewheels are still available, and not expensive, and will have modern tooth profiles for better shifting performance than the OEM freewheel.
They have gotten a cassette that they claim will fit, if they can get the freewheel off. I'll let you know how it turns out in a few weeks.