I have a older MTD Yard Machine that will jump out of gear on a pull, when this happens it throws the belt off. I was told that it could be a bad pulley so I replace the belt and pulley with new. I mowed about 3 times with no problems, the last time I mowed I could not keep a belt on, every time I came to a hill or put the mower on a pull, it would throw the belt and jump out of gear. I could put the belt back on and go to take off and it would throw it right back off, if I pushed the mower to a flat spot, I could go until I put it on a pull again and it would jump out. I ahte to scrap it becauise everything else works fine, any ideas greatly appreciated?