My Cub Cadet is acting up lately. Whenever I put the "single-lever" height-adjuster in the top/highest position, it doesn't want to stay put. It will stay for a short while, and then either jump to the next lower position all by itself, or when I engage the self-propelled forward motion from a standstill it will kick itself out of the top position at the same time as the wheels start to turn. any ideas?