Pedalling Problem

[biggs682]

i would also suggest that the freehub or rear cog set freewheel has seen better days

 

[Dan B]

I had these symptoms once, and the problem turned out to be that the lockring on the cassette had come adrift and the top two sprockets wiggled sideways off the end of the splined section, allowing them to spin freely. That's probably a less likely explanation than the other one you've been given (bad indexing so the chain is not engaging on the cogs properly), but worth checking anyway.