2004 impala ss stall

I have owned the car for 1 year now the car has 32,000 miles and is rarely ever driven the only maintenance the past year were oil changes tire rotations and battery but its now freezing temps outside and the other day I decided to drive the car let the car warm up as usual it did fine and sounded ok idle was normal once the car was completely warm I put the car in reverse and the rpms dropped into the 300-400"s and acted as though it wanted to cut off but the rpm"s rose to about 2000rpm and then leveled off and the car drove fine and has repeated this after the warm up yesterday but 2day I didnt let it warm up completely and it didnt do it just realized that the air filter hasnt been changed in about 9000 miles nor has it had a AIC control cleaning of any sort please help.