top end acceleration issues

[Graham]

Bike starts up fine, acceleration sounds normal, but when I top out It sounds as if the engine is struggling in an odd way. It vibrates a lot and sounds like the rpm goes out of whack what could this be contributed to?

[Spunout]

check to make sure all your hardware (esp. mounting) is nice and tight.

EDIT: if it isnt that, you may have a cracked frame and not know it. i've had that one before.