Tire Valve problems of the 2005 Ford F-350

We have a 2005 Ford f350 super duty. On vacation in September 2007, we had 2 valve stems fail from cracks which caused flat tires. The first was as the truck was parked. The second failed as we were on the interstate doing about 60 miles per hour. We heard it blow and got pulled over with no problem. We had both valve stems replaced and discovered that a third one was also cracked. We subsequently replaced all five valve stems with metal stems.