When I try and accelerate hard I get a surge in revs, just a little after a first power burst. With the surge in revs the power drops then catches on again. My guess is this is the clutch loosing grip. Do you agree? Happens in any gear. It still pulls off at fine initially from a stop, just with a little of that dry clutch sound until it grabs.

Yes Warwick,easy to check,friction disc most likely near their end. Make note of the order of parts when you take them out. No special tool required unless the basket is shot

Are you sure its not wheelspin ? ahhhh is your sprocket still bolted on