I have a 1990 Park Avenue 3.8L with 165k miles and has the Automatice Climate Control and the heat works really good and hot when im driving down the road but when i let the car sit at idle for a bit the heat becomes cold. Then when i start driving again it will get hot again. What could the problem be?

sounds like it's a little low on coolant. Worst case scenario could be water pump starting to fail.