We have a Cub Cadet RT65 tiller, about 4 yrs. old. Although it propels forward and reverse, and the tines rotate when engaged, they stop when lowered to the ground. If I lift the handles slightly, the tines begin rotating again but barely scuff the surface, and they stop if I allow the weight of the tiller to bear down. The belt is as tight as I can make it. Do I need a new belt, or does this sound like a transmission problem or something else? Thanks for any suggestions.