My hydrostat makes excesive noise and seems to be slipping as it gets hotter. A few years ago I put quite a strain on it while plowing, and immediately noticed more noise than before. The noise (sounds like air in the system) never went away. In fact this year it has worsened as well as a severe loss of power to the ground as the oil gets hotter. Have I hurt the Pump/Motor and if so where can I send it for rebuild? Suggestions, options wanted.