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?

Could be allot of things.

Of course when your engine is working harder its generating more heat. A driven car heats up faster than one sitting idling.

Other possibilities are vacuum leaks and when the vacuum is stronger actuates the vents better. Check all your lines for cracks and aging.

Your heat exchanger could probably use a good cleaning.