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this is not a very helpful response, but I had a similar issue with a new shimano 10 speed drivetrain recently. After riding it a bit, the issue went away. someone suggested to me that the mech hanger might be out of line. I never checked it though as it's fine now.
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As you say it's the middle sprockets, I'm wondering if it's a case of trying to do some extra fine tuning to get the indexing absolutely nailed? I'm thinking if the tuning is a hair out, then the first 3/4 sprockets would still index, then the final 3/4 would be the same because these ones tend to be widely spread ratios more than the others. The middle few would be relatively close ratio, and would show up the tiniest misalignment in the indexing. So you might want to try fettling a little more, aiming to get the chain positioned absolutely bang on centre of the smallest sprocket when clamping down the cable pinch clamp, which would obviously mean quite a few tries. I think it's made more fiddly due to the chain being the extra thin 10 speed type, although I admit I'm used to dealing with 8 and 9 speed setups more.
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Alban
walkmanman
@Thankshallots
I've put together a 10 speed campag chorus groupset but cannot index the middle sprockets on the cassette the chain always slips here. Only thing not new are the chainrings. Any ideas?