Clutch slipping at high reves, not all the time.

Clutch is slipping.

Sounds like the friction surface of the clutch disc is slipping. You would have to install a new clutch (clutch disc, pressure plate, and clutch release bearing), and have the flywheel surface cut and polished back to perfect smooth by a shop.
Manual transmission gears don't slip like automatics. And lack of fluid in the transaxle or the hydraulic clutch system won't cause this either.

Sounds like a clutch slipping.

the bad new is that the transmission has failed....the high revving is due to the clutches slipping (yes, auto transmission have clutches, about 3-4 of them wet), as the material wears on them they start to slip.

i bet if you checked the transmission oil, it would smell burnt and be dark brown, it should be red and sweet smelling.....no cheap way around this unfortunatley.....sorry

If the bike is reving high in top gear but you are not gaining speed or very little speed your clutch is slipping. This is a sign the clutch needs replaced.

If it is an automatic it sounds like it has seen its better days and needs replacement. If it is a manual tranny you have a worn out clutch that is slipping and needs replacement.

A bad fan clutch sounds like a real loud vacuum cleaner usually when going bad . What does your transmission fluid look and smell like ?