This mower was bought new last year and after a few uses the motor would slow down and then surge until it would just die. After a few minutes of waiting/cranking, it would finally start, run for a few minutes and shut down again. After a few weeks of this, it would not start at all. The spark plug is not fouled, so it is not at fault there. The engine is a 13.5 HP. What could be the problem?