Tire Valve problems of the 1998 Toyota Corolla

Two years ago I replaced all the tires on my 1998 Toyota Corolla with a set of new kumho touring a/s 795 (175/65sr14). The tires are now about 15,000 miles old. The warranty states they should last 70,000. On 01/05/2009 I got the first flat tire. It has been diagnosed by auto service as caused by a "bad valve stem". On 07/03/2009 I got a second flat tire. It occurred with a different tire than the first incident above. It has also been diagnosed by the auto service as caused by the valve stem. Today 08/08/2009 I got the third flat tire while driving on highway. I have not got yet to the chance to go to the auto service but it seems these tires are not very reliable.

Bfgoodrich 175/65/r14 tires purchased at sears without "road hazard" on 4/22/2007. Tires claim to have a 60,000 mile manufacture warranty. Tires have blown out on the highway on 3 separate incidents. The first time, service tech's were not able to determine cause- no road hazards. Purchased another brand tire. Second incident again on the highway. Car was taken in and service tech's again could not determine cause. Manager at sears sold me another tire (same model) at half off. Third incident occurred on the highway again. This time same manager at sears did not resolve issue, claimed it was a valve stem and because I did not purchase road hazard said it was not covered. I insisted that he check if it was a tire defect and he responded that he could not determine that. Left sears riding on my spare tire. Nothing was offered by the manager. All 4 tires have blown out now and I don't know who to turn to.

The contact owns a 1998 Toyota Corolla. The contact stated that all four tires and valve stems failed over the course of one month. The dealer stated that the valve stems failed, which inevitably caused the tires to flatten and fail. All four tires and valve stems were replaced. The contact has a copy of the invoices and is in the process of notifying the manufacturer in reference to NHTSA campaign id number 08t018000 (tires:valve). He does not know the information pertaining to the equipment. The VIN, serial number, and part number were unknown. The failure mileage was 62,977 and current mileage was 64,500.