the rear mech changes lovely up the cassette from smlest to biggest cogs, changing down it changes down fine for the first 4 cogs, then for the next 4 it needs 2 clicks to move down one and then an upshift to counteract the 2 downshifts. the last 2 shifts to use the 2 smallest cogs work fine. I thought at first it may be dirty cable but cleaned everything out and it releases the correct amount of cable for 6 of the 10 shifts
any suggestions is it likley to be the mide of the cassette is wearing out?
cheers
recently had the same thing, fixed with a new cassette, chain and a tweaked mech hanger.
Have you messed about with cable tension? Couple of turns of the barrel adjustor can make a difference.
have you check hanger alignment? (with the correct tool)
Is the chain properly lubricated?