flatspot

i have a 650gs 2001 with fuel injection, bought with a knackered gearbox.
rebuilt the box (broken selector), its runs great until you accelerate hard up to bang on 4,000 rpm, then you get a massive hesitation so bad you would think it jumped out of gear. then it pulls all the way up the revs no problem. it happens in all gears only under hard acceleration, if you accelerate normally its pulls perfect with no hesitation at all. new plug fitted still the same. its not a surge that you normally get due emmision controls as i am well used to that due to having an 1150gs then 1200gs.
any help or input would be appreciated.

clutch plates?

the clutch is fine as it not slipping, my fault is electrical. (almost like hitting the kill switch on/off for a split second)
no faults found with a gs911 diagnostic check.