Lay the bike down on it's left side with the fuel turned off. Remove the clutch cover and inspect the basket. It sounds like you have a notched basket. The notching prevents the plates from spreading apart and causes the problem you discribe. If it has not been done previously, you can file about 1-2mm of the notching away. To do this you'll need to drain the oil, remove the pressure plate, springs and plates. Don't file to a completely smooth edge AND keep the edge square. When you reassemble, flip the fiber plates over, they will have a sharp edge from the worn basket.
The best fix is a new basket, fiber, and metal plates but the notched clutch basket is your main problem.
The inner hub can be notched as well but from my experience the basket is your problem.