I was going to post the same question today. I often have that problem on my 1989 1680. Yesterday in very poor flax, when I would hit the better swaths my rotor would fall from 950 all the way down to 750 while the engine RPM fell very little. In desperation I found an old can of belt dressing and sprayed both the rotor belt and the big triple v-belt beside it. It solved the problem. What do I do nowIJ Do I keep using belt dressing, or does that mean it's time to change the belt or beltsIJ (I should have only done one at a time, to see which one it was, but at that point I wasn't thinking of that.)