Rear shifting woes - running out of options

My rear derailleur isn't shifting very consistently, particularly across the smaller 5 cogs on the 10 speed cassette. What I've tried so far

• checked rear hanger alignment with Park Tools alignment tool as well as removing it and checking it on a flat surface
  
• new inner cable and checked it was moving smoothly through all the housing
  
• cleaned rear derailleur and made sure it was moving smoothly
  
• checked wheel alignment and that the cassette was tight on the wheel
  
• adjusted cable tension but can only get either reliable shifting up or reliable shifting down but not both
  
  
  

I'm running out of ideas for what could be causing my shifting problems. I've managed to get it to the point where it shifts through most gears apart from a couple of the middle cogs where I need to move the shifter until it clicks and then give it a little extra nudge, before it gets to the second click, to encourage it to shift up.

By the process of elimination, assuming the cable and derailleur are fine then it must be the shifter, however it appears to click through all 10 positions fine both up and down. I've read about lots of cases of Shimano shifters jamming up and not clicking through at all but never of one having uneven cable pull between indexing positions?

Anyone have any other ideas as to what else it could be?

Did you check where the cable runs under the bar tape, pressure as you grip the top bar over time can squish the cable housing a little and that causes it to be "sticky". Long shot I know.

Apart from that maybe bent it slightly by catching the deraileur arm while chaining up or resting the bike against stuff. Wheel and cassette aligned but derailuer is just slightly out.

Maybe try new cassette & chain? Are they well used?

Another long shot but I've known it to happen - is the cable correctly routed through the anchor bolt?

Also, check mech cage is straight, and that the pivot doesn't have any play

^^ 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 ..

I didn't know this until the other day and it explained the weird meshing I could feel going on, some chains are directional, I can't remember which ones. When I changed my chain after it reached the .75 and put on a standard 10 speed one it felt all ok again. It might be worth checking because until I changed my chain I occasionally did get lazy changes.

Have you checked the cable outer? It sounds like it is a possible culprit, have a look at the ends inside the ferrules, are they nice and neat, or beginning to fray?
Are the ferrules all intact, if the cheap plastic ones they often break up under pressure from the cable outer.
Also check the cable outer is not corroding, and that the plastic outer is sound, with no splits.
They might also (especially the bit to the mech) be full of crud, which you can trying blowing out with a blast of silicone lube from an aerosol.

Sometimes the outer cable can fray a bit where it goes into the shifter. It doesn't sound like that tbh, but worth a check.

Like Skully said earlier, symptoms sound like worn chain/cassette.