Incorrectly adjusted/faulty throttle cable.
Blocked air filter.
Blocked exhaust.
Faulty Carb (Needle not lifting clear of the main jet).
Fault alternator/electronic pickup/advance retard unit.
Valve clearance.
Cam chain adjustment.
Something lose that resonates at those revs and screws things up.
Faulty rev counter (i.e. it is doing 9,000 RPM but only reads 7,000 RPM so you
are bouncing it off the rev limiter).
Exhaust starts to seperate at those revs and ruins the engine tune.
Old and worn out pixies.
Loads more I can't think of just now.
HTH
Clinton