Annoying indexing problem

[Steve vRS]

My rear mech has developed an annoying index problem. Every gear, up and down, shifts crisply and accurately. Apart from one, the 4th from top.

It refuses to seat, the chain just jumps around. It is a new chain. 10 speed 105 by the way.

Any ideas?

Steve

[Steve vRS]

Someone else suggested worn cassette. I didn't think I used that gear disproportionatly more than the others in the middle of the stack.

Steve

[Steve vRS]

First chain but it is on a CX bike and gets ridden in mud, rain and other questionable substances!

Steve

[Steve vRS]

Would misalignment not give problems across the cassette? I just have one troublesome gear.

Steve

[Steve vRS]

Well I've tried a new cassette as that was the easiest fix and there is no difference.

Cable next? Or I wonder if anyone nearby has a hanger alignment tool I can try...

Steve

[Steve vRS]

This bike has internal cable routing which scares me so I've dropped it off at the LBS to sort out! It also has a dodgy TRP caliper which was just replaced but has foofed up again. Grrrr

Steve