"False shifting" I'm stumped...

[I am Spartacus]

Just an aside... you have purchased gear cable as opposed to brake cable?
They are not supposed to be interchangeable.

 

[palinurus]

Does it only happen under load or does it shift down even when you've got it on the workstand/ hung from a tree or however you do it?

 

[MartinC]

Amongst all the other causes this can be triggered by the cable binding in the guide under the BB - frame flexing can then tug the cable. Clean and lubricate to fix.

 

[Panter]

Just an aside... you have purchased gear cable as opposed to brake cable?
They are not supposed to be interchangeable.

They are gear cables, I believe brake cables are thicker but I'm probably wrong

Does it only happen under load or does it shift down even when you've got it on the workstand/ hung from a tree or however you do it?

Originally it did both, which I thought (and still think) was down to a dodgy cassette.
This time it's doing it in the stand, I haven't road tested it yet as I don't want to risk damaging the new cassette with the chain keep jumping to and fro

Amongst all the other causes this can be triggered by the cable binding in the guide under the BB - frame flexing can then tug the cable. Clean and lubricate to fix.

There's no dirt under there at all, it's been thoroughly cleaned and it's a new cable.


Thanks for the ideas all, I'll post back if the new cable fixes it or whatever the LBS comes up with.

 

[normgow]

There might be one or two rivets on the chain standing proud of the links. A careful adjustment with a chain riveter could help.