Is it a new cable? If it's an old worn one then a frayed inner could be getting caught. Also have you checked that the derailleur isn't getting too close to the cassette?
Yeah, new inner and outer. I've tried it with and without a derailleur extender, doesn't seem to get close to the cassette in either setting. I have a feeling that the shifter has somehow pushed its indexing several stops forward, but I'm not sure if that's possible/makes sense...
Is it a new cable? If it's an old worn one then a frayed inner could be getting caught. Also have you checked that the derailleur isn't getting too close to the cassette?