Won't rev past 6k rpm under load!

[Robbie8666]

as title, bike been in workshop for nearly 3 months now & mechanic no further forward!!
HELP!!
when not in gear bike revs all the way to red line
in gear the bike pulls & revs smoothly up to 6k in all gears but won't go past that
mechanic bought Yamaha's manual direct from Yamaha has checked all voltages etc
he has changed ecu
spark plugs
HT leads
stripped & cleaned fuel system pump filter
stripped & cleaned carbs
balanced carbs with electronic tool and compared against each other
checked coils and all with in tolerance
check clutch for slippage
has checked side stand switch as that was suggested by someone


a) has anyone had this & fixed it?
b) any other ideas where to try next!

I have got to the stage where I said i'd sell it but he reckons with the problem unfixed I'd only get £450 so not going down that route!!
missed a lovely Fazer thou so decided to try & fix this one then when I find the right replacement keep this as a 2nd bike / one for Mrs use

any ideas? getting pee'd off as missed summer again due to not having my own bike!!so 2013 missed summer 2014 rode 2015 missed summer!!