Tire Valve problems of the 2003 Hyundai Elantra

I purchased new tires for my vehicle in 2006, I drive only about 8000 miles a year. I have had two flat tires this year, one on 5/21/08 and the other on 8/31/08 and both were cause by faulty valve stems. With the incident on 8/31/08, my tires were all inflated properly when left I the house to run errands. I left one parking lot, turned on the main street and within seconds I knew something was wrong and I had a flat. It happened all of a sudden. The flat was fixed (roadside assistance) and I then went back to sears where I had purchased the tires. I had the roadside hazard agreement. The technician at the auto center said there had been a problem with valve stems. I saw something on tv about a horrible crash that happened when the very same thing happened to some man. I was lucky, I had only turned onto the main street and was not going very fast.