Advice on motorcycle problem, my CB200 runs fine on start up, but when warm it has a major flat spot ?

I have changed the coil, leads, plugs, condenser, timing is spot on, carbs are clean and with new jets etc. When you get past the flat spot ie full revs, it runs fine but as you shut off or when you try to increase revs steadily it dies. When its cold it works fine , but as it warms up it goes stupid. I'm stumped so help please