^^ This .. I not saying you've done this. But I've fixed DIY "it won't shift properly now" problems by simply routing the cable correctly at the mech and the clamp bolt. If you have fitted it incorrectly what's happening is you've changed the pull ratio ever so slightly so you can get part of the cassette shifting nicely then the accumulation of error starts to stack up and the shift goes to pot ..
^^ This .. I not saying you've done this. But I've fixed DIY "it won't shift properly now" problems by simply routing the cable correctly at the mech and the clamp bolt. If you have fitted it incorrectly what's happening is you've changed the pull ratio ever so slightly so you can get part of the cassette shifting nicely then the accumulation of error starts to stack up and the shift goes to pot ..