So, the weekend before the 24/12 I realise that there is a kink in the inner cable, between where it exits the rear mech & the clinch bolt that holds it tight and I can't get the shifting right….as I shifted from small cog to largest, the kink got pulled inside the outer cable & then moving from largest cog to smallest, the kink is causing the cable to snag, and the gears won't change until there's sufficuent tension in the mech spring to overcome the kink. When the kink was situated with no outer around it, shifting was fine for that limited range of gears.

I fitted a new inner & within about 5 mins had the rear mech running absolutely sweet.

Now….a few weeks weeks later, I thought I'll give the bike a clean & get in a few hrs on the road. Clean the bike…..check the gear shifting before going out & it's not great so I try to sort it out. It's now about two hours later & the shifting is better, but not great, it's now too late for me to go out for a ride & I am thoroughly cheesed off with the whole thing.

This seems to happen about once every month or so with my bike. I don't know if it's crap in the cable, the mech is bent, the cassette/chain need replacing or what……
I am inclined to think that it is the cables as they are not as smooth as they were when new, but they are by no means 'sticky'. I don't think it's the mech or worn cassette/chain as it was running absoluetely perfectly three weeks ago. I don't know if my bike is particularly susceptible to getting grit in the cables or what, but I can't afford to put new cables on every 4 weeks – no one else I ride with seems to have these issues & we ride in the same conditions. I did fit a complete outer for a while to the rear mech, but as there is no easy way of fastening to my frame, this was more problematic than it was worth. I don't know whether to try some of those sealed cables systems…?
My shifters are really old & I am wondering if they could be partly to blame?
I don't wanna start changing bits just for the sake of it hoping to find a solution – I would like to identify the problem without just throwing parts at it. I know if I take it to the LBS, they'll whack a new cable on, not tell me what else they've done and it will be fine for a while & then crap again.

Where do I start to identify what is causing this? It all seems so linked together that I am not sure how I can isolate the problem, or whether it is just my poor set-up. As you can probably tell it is driving me absolutely mad & everytime I have poor rear mech shifting I know it's going to be the same scenario.

I have used 3 online guides to get to a position where the shifting is not too bad now, but it is still not perfect. It shifts OK across some of the gears, but not all. AAAARRGGHH!

And please don't mention singlespeed….I am far too unfit for that.

EDIT – oh, and that B-tension screw doesn't seem to make any difference at all to the position of the rear mech…..

I run a sealed system whee the inner cable is in a thin plastic outer cover that runs inside the outer, if you know what I mean. It works perfectly.

Gore sealed cables.

Is the mech wobbly?

OK, Full suss? are the cables too short. What do you mean by crap shifting, dragging up/down the block? Or just not shifting at all? How old is the chain/cassette/mech? Is the cage on the mech straight.

Check your dropouts are straight.

Troutwrestler….that's what I am considering trying. The Flying Snake ones seem to get a good write up.

Daffy…as above & no rear mech is not wobbly – no play in cage & it all looks straight – not had it that long.

NickC – yes full sus & I haven't changed the cable length since they were fine a few weeks ago, so I am pretty sure they aren't too short. At the moment I can't get them shifting great even on a workstand, so I don't think the suspension action is having an impact.
Main problem is shifting back down the block – large to small. It's similar symptons to the kink in the cable I mentioned at the top, so it could be resistance in the cables again. Perhaps sealed cables would be a reasonably cheap first stab at sorting it out.
Chain and cassette have been on a while – should probably change the chain really. The mech is < a yr old.
Cage on the mech looks straight.

Mackem – dropouts look straight as far as I can tell. I haven't done much riding since the 24/12, but it was REALLY wet & gritty at Chicksands on Thursday so perhaps that's pointing in the right direction.

Been looking at new shifters as mine are quite stiff, old & generally not feeling too good – particularly the rear – but I can't find just a rear SLX shifter. CRC has them in their stock list, but is out of stock. I thought that you could buy shifters individually, but is that not the case?