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cycleclinic
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Jeff 80.
The jumping problem sounds to me like an incompatible chain and sprocket rather than a malfunctioning derailleur. Even the best modern mechs won't stop the chain jumping when there is wear on the chain or the sprocket, and jumping on one sprocket is typical, since it's the most worn one that jumps.
Of course, if you're using a new block and a new chain then I'm wrong, but if one is new and the other is not this could well be your problem.