Originally posted by mike
I'm with Alex*D and ChrisL.
Is your bike mechanic reliable/trust-able? Look at your sprocket teeth. Do they look like shark fins or are they really pointy instead of flat topped? This is the sign of worn sprockets.
We can guess that only one or two sprockets on your cluster are worn. You should be able to replace these sprockets rather than replacing the whole freewheel/cassette.
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They are a good LBS generally (one of the best in town, I think) and I don't think they are a rip off. You do make a great point though. The sprockets teeth are needed very worn but not on all of them. Perhaps replacing one or two sprockets is the way to go.