Tire Valve problems of the 1998 Dodge Grand Caravan

The contact owns a 1998 Dodge Grand Caravan. The vehicle has michelin x radial tires. While driving 45 mph, the contact heard a noise coming from the rear of the vehicle. She noticed that the rear passenger side tire was flat. The tire shop stated that there was a crack in the valve stem and they made the repair. Approximately six months later, the failure recurred on the front passenger side tire. The tire shop stated that the valve stem was cracked and they made the replacement. There have not been any other failures since that time. The current mileage is 130,000 and failure mileage was 129,000.