It's driving me mad on my roadrat. There are no problems at all when I am sitting but as soon as I am out of the saddle the gears are all over the place. I am running 1×9 with a 50 tooth compact single chainring at the front and an XT shifter and cassette at the rear. I think there may be some problems with excessive cross over, but it happens all over the cassette and makes riding in the woods a nightmare.
Any ideas?

indexing is out.

cables may be a bit sticky?

cross over certainly wont help

try full length gear outers …on some bikes (eg my old nrs) frame flex can cause the problem. A full length outer sorted mine.

I never seem to have a problem with phantom shifting despite putting my 105kg lump on an NRS and mashing the pedals?

worn chain and cassette first option after indexing not quite being spot on. I too found the easiest way of sorting it is running full legnth outer.

To be honest as soon as I get problems like that I'll change the cassette and chain, look at the chainrings and change the geat cable too. I can't stand bikes not doing what they're meant to do quietly and well!!