shifting problem mtb
having some trouble with my mtb shifting and need some help. btw, i am a decent mechanic, but not an expert. the set up i am running is shimano xtr m 966 dual control brake (hydraulic) / shift levers and xtr m 960 rear d. i have a fairly new xtr/dura ace chain and gore ride on cables. the cassette is older, but looks good. the bike shifts all gears great except for the problem i am about to describe. the chain will not stay on the second to smallest cog when i am up OR down shifting. going to a harder gear, the chain will shift to the cog and then go back to the 3rd smallest cog when i let go of the shifter. going from the smallest cog the chain shifts and the automatically shifts again to the third smallest cog. the chain will stay on the second cog during a shift if i hold the lever down. as soon as i let go of the lever the chain will go to the 3rd cog. everything else in the drive train works fine and is quiet as can be. i actually want to use this cog as it is the perfect gear for going fast on the road when riding to the trails. and last, but not least i would prefer to not buy a new cassette. help.