Did you try adjusting the cable up on it? It probably does it worse when it's cold, right? If the cable is adjusted right, then it's probably the clutch basket. Those clutch baskets are made of aluminum and after a while the clutch discs wear grooves in the slots in the basket and they want to hang and not release properly. You can fix this by taking the clutches out of the basket and taking a flat file and filing down the notches in the sides of the clutch basket. Just file the notches down until the sides are smooth again, it won't hurt the clutch basket. Try adjusting the cable first, but I'll bet it's the clutch basket worn.