Outer Gear cable - to replace or not?

Yes

Have you checked mech hanger alignment?

The more cogs on the back, the more sensitive it is. The bigger the cogs on the back, the more sensitive it is.

Even new mech hangers come out of the packet misaligned 

@Onzadog I don't have an alignment tool but sighting it, it looks fine and moves up (to largest cog) the gears nicely just not back down.

If you adjust cable tension so it comes down okay, I assume it's slow going up?

Could be sticky housing, but could still be alignment.

Sighting isn't much use as it only needs to be out by a gnats bollock.

Change the outer and inner (again). Use descent quality cables, ie. Shimano. It's probably drag in the system that is stopping the mech pulling the slack through (not moving enough).

And check that hanger with a proper alignment tool.. it's unlikely to be 100%

Replacing outers is an annual preventative maintenance task for me.

You need an alignment tool now we are using huge cassettes, needs to be aligned accurately and even new hangers are not always straight once you've bolted them on to your painted frame or tightened your axle which moves them.  This is the UDH hangers have been introduced, as hangers have such bad tolerances.

What alignment tool (make and model) do people recommend? The Park Tool is pretty £££. 

I use an Abbey Tool HAG, if you think Park is spendy, brace yourself.

However, I started out with a Cyclo 9ne and got exactly the same results. Even if the cheaper ones feel a bit rattly, so long as you keep a constant pressure on the either directly towards, or away from, the bike, you get consistent results.

https://velo.outsideonline.com/road/road-racing/the-best-derailleur-hanger-alignment-tool-9-tested/

I have the Lifeline one which works well enough but just invested in a Park DAG-3 which came up in a recent sale. Much quicker to use and more accurate.

They really are essential tools for eliminating shifting issues on 11 / 12 speed setups so money well spent of you do a lot of spannering.

I'll always replace the outer when fitting a new inner cable.  But I am lucky that my bikes pre-date internal cable routing.  I can see why folk don't do it.

I’ll always replace the outer when fitting a new inner cable.  But I am lucky that my bikes pre-date internal cable routing.  I can see why folk don’t do it.

Ebikes are even worse:  Just replaced the gear cable on my Orbea Rise... had to drop the motor as the original cable was trapped by it. Had to remove chain ring to get at motor mount. Had to buy new tool for ebike chainring. Second one fitted. Realised that rear pivot had a bit of play. Ended up refurbing whole back end. Talk about mission creep!

Yes replace the outer. The ends get a bit tatty where the sleeve comes away from the sheath and can lead to funny shifting 

Yup.

One of those preventative maintenance / just get on and do it whilst you're there, jobs.

Buy several meters of SP41 outer at a time, buy a couple of packs of shimano ferrules, the noodle ones for the mech end and any open bits on the frame, and the plain ones for the shifter. And a load of inner cables. Then you can do it first whenever there's an issue as 9 times out of 10 it's the cable/outer that's knackering the shifting, and it's rarely worth trying anything else until they're changed.

n.b. this holds true IME, but maybe not always:
*Bent hanger tends to mean it won't shift at one end or the other, either gears 1-9 will index, or 4-12 will index, but not all of them.
*Bent mech won't shift to one end of the cassette depending on which way the parallelogram is twisted (can be really hard to spot by eye and needs a new/spare mech to try and see if it resolves it.
*Knackered cable, starts off struggling to upshift/release cable in the middle of the cassette and get's worse from there.

A longer outer for the last cable segment into the RD can help, but I will always replace this as it’s where the friction builds up. It’s a very frustrating issue sometimes.

I have the lifeline alignment tool, there is a bit of play but as you just need to gently hold it towards the rim when setting the distance guide, and then when you rotate it 90 degrees, gently hold it towards the rim again.  I think its fairly each to remove the play, I'm sure I read someone here saying they had fixed it with a few wraps of tape or a blob of grease or something.  Remember that you should rotate the wheel with the hanger, this removes any error caused if you have a slight buckle in the wheel.  I always keep the valve rotated to match the tool.

Incorrectly adjusted b tension will also give those symptoms. 

And wobbly jockey wheels

Sounds like the mech needs a service to me.

IME it's the last few mm at the derailleur that rust where the outer is cut. You might get away with trimming back to clean metal if it's a continuous outer and pulling it through to reinstate the loop at the rear

Then as above ^^

Also if routing is internal it's handy having one of those £13 routing kits you can pick up off Amazon. Makes life so much easier for maintenance.

...wobbly jockey wheels

The upper jockey wheel (guide pulley-marked G) on a Shimano mech is meant to float a bit, wobbling is a problem 🙂

If it's a Shimano mech is it better with the clutch switched off ? they can get sticky but are easy to service

2nd what stue says. Shimano clutches often get sticky and stop the mech shifting to smaller sprockets.  If it shifts o k with the clutch off then check that the clutch isn't adjusted too stiff,  if it's fine then get some of the shimano clutch grease and give the clutch a clean and regrease.

Also, if you've fitted a new inner, just double check you've got the cable going around the mech pinch bolt the right way.

Thanks for all the responses.

Bit of a update, after lots of faffing I have replaced the outer and it's working better but still have issues coming back down (large cog to small) and it's definitely worse with the clutch on. 

So I will get the derailleur off and give it a service including the clutch.