I agree with Restrorob. Last spring I had a similar problem with a Dixie Chopper that had a stretched belt. The deck to motor belt would vary in tension as I raised and lowered the mower deck. I was convinced it was something more involved than just the belt, seeing as it had worked fine prior to parking it for the winter. I later figured out it had stretched out from contact with excess grease from a spindle. New belt fixed it.