Tire Valve problems of the 2004 Nissan Altima

On December 01, 2006 I purchased 4 new tires big o t/r euro tour p205/65r16 dot id 3d3pt1n2906. These tires carry a 4 year 70,000 mile warranty. Within 5 months and only 3758 miles (April 2007) I began to have tire pressure problems with the left front tire. I returned to the big o tire store and they found that it was a bad valve stem that they had replaced when I bought the tires and repaired it. In may 2007after 18 months and 26596 miles I began to have problems with the tire pressure in the right rear tire. This time they found that the tire had a separated belt and replaced the tire. At this time I tried to get them to acknowledge that they had a problem with this lot of tires and get them to replace the other three. They refused to do so, saying that if I had other problems they would take care of it when it happened. Today January 26, 2009 I went in to the big o tire store to have my tires rotated. After they had finished as I was on my way home my car was pulling to the left. So I went back to the store. After some trial and error they have found that a second tire that had originally been on the right front has a belt separation problem and are going to replace it. Since two of the original tires have failed in the same way I have asked that they replace the other two original tires before I have more problems that may be more severe. So far the answer has been no from the local store and from their national consumer contact. My view is that this is a safety issue not only for me but for others.