2003 Dodge Ram A/c problem-not cool when sitting still

My air conditioner is blowing hot air when the turck is parked or sitting for a while at stop lights. It works fine and blows cold air whenever the car is in drive. It has been over 100 degrees the past few days since I've encountered this problem. Just wondering if wnyone could help. like to fix my cars rather than take them into a shop if at all possible but not sure about air conditioning. Thanks.