Motorcycle Questions? Ask a Mechanic.
A little more information may be helpful.
I see. The problem is likely a slipping clutch. The clutch will slip easiest in 6th gear. It is really the only component that can cause the problem if it only happens in high gear. Most any other issue would manifest itself in any gear and speed.
You may simply need an adjustment or you may have worn friction plates. There are several reasons they can slip.
Typically, if this is the issue, you can smell that the primary fluid smells somewhat like hot brakes smell (this is the only way I can explain it).
I hope this helps. Please let me know if I can be of further assistance. Thanks, P.J.