[Closed] rear mech set up

ahhhh..
have set up rear mechs countless times. other night chain started climbing highest three cogs on rear cassette, climbing up dropping off. right pan in the arse especially as it was 10oclock at night and i was ontop of boredale hause.

Anyway i've set up rear mech from scratch and it's still doing the same.
chain at <0.75%
high/low/b screw all set up
rear mech/lever barrel adjusters make no difference.
cable cleaned, mech and cassette cleaned..

anyone got any ideas/suggestions?

?user error.

It's driving me nuts.