Tire Valve problems of the 2003 Chevrolet Impala

I bought 4 new tires at sears in wlilmington, n. C. Last may, I have put less than 12,000 miles on these tires. Beginning in ;April of 08, I started to notice a low tire and proceeded to the station to pump it up. As I was putting air in the tire I heard air escaping and moved the valve stem and it was loose. I put enough air in it to get around the corner to the service station and he said it was the stem. I was trying to decide how I was going to afford a new tire and he said that he could replace the valve stem. He charged me $10. This happened about three weeks later and then again this week, may 21! three tires out of four have had the stems replaced. I have been driving over 40 years and have never had this happen to a tire, I didn't even know the stems could be replaced. I called sears and they told me to bring a receipt for fixing the tires and they would refund the money.