This mowing season, I have noticed that if I hit any kind of thick patch of grass/weeds or mow over some good sized sticks, the blades will slow down. In the past the mower has been a real bull and mowed thru anything that got in its path. Its not the deck belt because I don't hear it slipping on the pulley's. The engine doesn't choke down, at least not that I can tell. That leaves the possibility that the PTO is failing. Is there a way to trouble shoot this? What should my next step be?