I had my bike do this for awhile. Confused the hell outta me.
When I ended up doing was tearing the tranny apart to inspect the clutch (it was fine) and putting everything back together. But when I did, I first cleaned everything really well, and then refilled the tranny with all new oil.
The last owner had taken apart the transmission without knowing what to put in it afterwards. As such it was running with something far too thick in it.
If you havent fully drained your oil yet, that might be something to consider. Something about the two types of oil not mixing right or something...