We had the same problem when the mower was like a year old... very frustrating.. still under warranty.. Warranty guy was ready to move in with us and kept ordering parts, he finally ordered a whole new deck. When it came, the problem was obvious, the one pulley was bigger... he dug out the new bigger pulley wheel... put it on... and we have not have not had a problem since 2009.. We ended up with two new belts, all the pulleys, plus a brand new deck we still have in the box. He said it would cost more to ship the new deck back then it was worth. We have more in the mower then we purchased it for in all these parts. Apparently, they had a mix up on the line and who knows how many decks were assembled with this smaller pulley wheel on it. Good Luck.