These cars are notorious for having evaporator leaks. These are slow leaks and a recharge may last a few months to a year depending on the size of the leak but eventually the evaporator leaks out the refridgerant. Replacement is about $1100. When I had this problem my recharges would last anywhere from 3 to 6 months. When it got down to 3 months, I added a rubber and metal sealant and that seemed to seal the leak. It was a cheap fix, I spent about $50 for the Interdynamics kit and sealant kit. Just remember not to overfill the system, it'll shut down the compressor.