Why does my heater work better when my 92 Astro is under power?

Ok I am the proud owner of my 1992 Chevy Astro "AKA - Squeaky " which I use to go to college and on the weekends. While she may not be the pretty girl on the block, I still love her. Lately she has been having a problem retaining the heat level coming out of the heater. When it is under power and making it's way down the street ....heat is good. When she stays at the light or stops in one spot too long the heater actually cools down to the point it actually kicks out cool air??? The heater core is not leaking nor do i have any funny smells when the heater is on. Please help as it is getting colder and i would like to stay warm when she is stopped or at the light too.