First thing would be to drain the oil and note if it looks dark or black or smells burnt, then pull the clutch cover and work the clutch lever while watching the clutch pressure plate, note if it's moving in and out freely, also note how much lever movement before you see the plate start to move, then pull the clutch apart and inspect the clutch drive plates for warpage,the clutch basket for notching and pressure plate for signs of overheating/warping. Chances are your problem is in one of those components.