OK, here's the deal. I have a 93 5th Ave. with a 3.3. With the heater temp control all the way to hot the heat only blows at about 100 degrees as long as the car is moving or the engine is revved up. Stop for a light and you can feel the air temp drop. Start driving and it goes back to 100 degrees. The heater mixer door is controlled by a cable. Any thoughts or ideas are appreciated. Thanks in advance.

 

Low on coolant............

 

Sorry, I should have included that. The radiator is full and so is the reservoir. My thinking is leaning towards the water pump but, then it should overheat. That only happens in warm weather when going real slow or stopped at a long light.