have you replaced the chain and cassette and chain rings?
tried 3 chains, 2 crank sets, 4 cassettes. all of which work perfectly on other bikes.
Mech and mech hanger ok. Seem to remember the hanger has tiny screws that hold it in, could work loose. Or the hangers flexing. Hard to say without having it I front of me and having a tinker. Great bikes though!
tried 4 different mechs, as above all work perfectly on other bikes.
mech hanger is brand new as the tiny screws on the previous one had stripped threads so wouldn't hold. i'd be supprised with the flex of the hanger it's one of those nice expensive CNCd ones.
all the linkages are tight, no lateral movement in them.
as i say i've been through all the common causes of why chains skip and am at a real loss.
the only thing i can think it maybe is the cable outer that runs under the BB.