james01 wrote: I'd still suspect some kind of lateral movement in the block - perhaps only detectable under load?
Yes, there is some lateral movement here: about half a mm. The cassette is tight on the freehub, but removing the cassette shows that the freehub itself is a bit loose. Researching this for a Shimano freehub suggests that it should be some play here - but no values mentioned.
Anyway, rather than doing tests with limiting the indexing range as suggested, I just changed the whole rear wheel, swapping in my road bike wheel (with its 11-speed cassette ) and going for the same test ride resulted in only two 'autoshifts' and they were both positive, i.e. they changed and stayed put. And one of these I knew it was going to change because there was that familiar grinding noise just before. These shifts are not surprising running a 11-speed cassette with a 10-speed chain and shifter.
So it's clearly nothing to do with shifters, cable, derailleur or frame! it comes down to the wheel or its hub or freehub. I'm not going to start messing about with the freehub; I'm going to buy a whole new wheel, I was thinking of some lighter rims anyway....
