Incorrectly adjusted/faulty throttle cable.
Blocked air filter.
Faulty Carb (Needle not lifting clear of the main jet).
Fault alternator/electronic pickup/advance retard unit.
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.