My old gravely belt drive WB has something happening with the blades. When I engage the pto the blades are spinning but it doesn't seem that they are spinning at their fullest potential. Then today I was mowing a thick lawn, and the blades would slow down almost to a stop where I had to crawl to finish the yard. Is this a belt tension issue? Where else should I look for. it is weird because sometimes the blades sound like they are spinning super fast and other times lagging. Any ideas?