Thanks Tbran, Johnboy and Acre eater. The rotor slows in heavy crop but quickly recovers, once you slow ground speed.. I suspect this is slipping. The mchine is a 1987 model. When we cleaned the upper sheave around 2 seasons ago it was really full of dirt, we also noticed that at some prior time (we purchased 2nd hand) the clutch must have done a fear amount of slipping as there looked like a fear amount of wear. I am not sure how hard the belt is pushing against the sheaves. You can squeeze the belt together in the middle and it doesn't pull back.The belt is not too shinny. last season had the o'ring in the gear box repaired (cylinder side)as it was leaking.