1987 Chevrolet Caprice Clic Brougham Heater Problem!?

When driving car the Heater Blows Hot Air , But when car Starts to slow Down or is Idling the Heater Blows Cold. Can someone explain to me what can be the Problem? Car has only about 55k miles,has a 305 V8 5.0 4bbl Carburetor. Thank You for any Info Anyone can provide me with.