I bought a hustler fastrack 20 hp kohler courage on 9-6-05. Mowed with it for the first time today and the engine sound like it wants to shut down but keeps running. It is not because of the safty switch in the seat bacause I tested that already. It just sputters then keeps going. Sounds exactly like my butt bounced of the seat and wants to kill it, but happens when sitting still also. What should I do?