my chain is jumping around on the middle 3 cogs of a 9spd set up.
no amount of tuning will stop it.
when the chain is in each of the 9 cogs whilst on the workstand it works fine and dandy.

any ideas?

Something’s worn.

Is this the new bike Ton? What shifter/mech/cassette are you using?

new chain on worn cassette?
chain & cassette worn out completely
rear mech or bent
just guesses, very difficult to diagnose without seeing it.

How old is the drivetrain?

new bike. using road version of flat bar shifters.
but it was doing it with the drop bar sti units too.

Most likely cause is a slighty out of line mech. It only needs to be out a tiny bit. Even a new hanger can need a tweak to get it right, also check you tightened it to the frame properly, mine came ever so slightly loose which caused a similar thing. Check that first, then chech the other things metioned.

+1 for another “mech out of line” answer.

Or try the cables, they may be dry in the outers and a good lube my just release that teeny bit of snag that could be making the inner “stick”.. Worth a go IMO.

Another, check the cable routage, have you a cable carrier thingy under the BB? Lube that too..

Given that it seems to happen when riding it therefore when chain has load applied I would strongly suspect the chain/cassette. You say new bike. Is this new out of the box never been used or newish, ridden and been okay for a while but now has the issue.

ok for first 2 rides then started.

Cassette locking tight?

Shimano chain? I had a joining pin that had sheared leaving a protruding bit which rode up on the next biggest sprocket (like a phantom shift). That was ten speed though. Or have you had a stone strike on the chain, opening a link slightly? Seems odd it’s in the middle of the block though (straightest chainline) my troubles’ always shown up on the smallest sprocket.

I had similar a few weeks ago(bottom 3 cogs). Changed to a new cassette and it was fine after that

Ok in workstand, but not under load.

Does it matter which chainring you’re in? Is the chainline right?

You’re an old soldier so I assume you’ve tried pulling the chain backwards off the offending sprockets to check for wear. Pull it forward off the chainring too and it helps indicate whether it is the chain or sprocket at fault.

Had something similar once when I put the cable through the clamp on the mech slightly out of alignment (effectively got slightly wrong pull ratio) but I’d expect that to influence top or bottom sprockets not middle – and if you’ve changed shifters then you’d have had to have done it twice.

did a bit of tweaking earlier, nipped the lockring up, checked the dropout, adjusted the cables and checked for snagging. all good.
just been for a bit of a spin, and it seems fine…..we will see.

cheers all.

Glad you seemed to have got it sorted.