Just try running the bike in a very dark place and make sure your not getting any arcing along the plug leads over the rev range you have currently.
no cracks on the inlet rubber
also check that the back of the inlet manifold is flat I've had several that where not.
try running without plug caps
wire ignition leads
no loss of water from the cooling system, I've had left hand liner leak slightly causing excess moisture in cylinder at higher revs
exhaust is clear and not choked up, take off silencer and rev the bike, does it rev better
correct ngk plugs not copies you can get from a certain site
equal compression from both cylinders
primary chain correctly set, not causing a hunting which will affect ignition timing,
check the main choke tube is it fitted in the correct plane, i.e cutout nearest engine side
as Paul might have mentioned, is the choke seating correctly
a good electrical earth
not much else to think of at the moment, let us know how you get on
Kev