The reason the clutch works ok when cold and then starts slipping is that when the port is blocked, and brake fluid gets warmer and expands, it has nowhere to go except to the clutch master cylinder. You probably noticed when you first started riding, the clutch lever had some play in it, then as you rode the play got less and less and then none. I agree with @videoarizona, time to completely replace the fluid. I removed all the fluid from the reservoir of the Goldwing and with a small glue brush cleaned the inside completely with fluid and then wiped it out with a clean rag, then flushed the line with new fluid and topped it off and bled the system. I will bet there are many bikes out there that went to the dealer and got new clutch plates, and the mechanic also cleared the port!!