Carburetor problems of the 1998 Mercury Villager

1998 Mercury Villager purchased used in 2003. Idle throttle body failed in 2006, Mercury dealer took two weeks to find the part and replaced at my expense. Idle throttle body failed again in early 2007. Dealer took two weeks again to find part and did not charge me but I had to rent a car at my expense. Today 9/30/07 the same symptom of the car accelerating to 1500 rpm intermittently while sitting stopped at a light has begun again instead of constant 800 rpm when this part is working correctly. I will have to wait on the dealer to find a part and repair it again at my inconvenience and expense. Have other owners of this vehicle engine experienced the same repeated safety and waste of fuel issue?.