Over-rev?

Never had that one. Maybe a broken clutch spring.

Thanks
Randy

It almost has to be belt slip so what's the cause?
Check the belt for glazing-sides get hard surfaced from slipping and heat so slipping will get worse. If it's glazed you can use 120 grit sandpaper to remove the grit or carefully use a wire wheel on a grinder.
Too much grease on the upper jackshaft bearing can end up on the belt so check and remove any grease you find.
Your belt could be the wrong length, worn out of tolerance & or your belt deflection could be set too tight-if any thing was changed from last season you might check.
Have you serviced your driven?
It could be your weights starting to hang on the spider. When a clutch gets enough miles the weights wear against the side, bushings wear and eventually the weight will hit the spider. When that happens it slows upshift, might increase R's, you will feel a surge as they stick and then shift.