Intermittent Heat From My Heater

Hello All,

I have a problem with the heater in my 92 E36 325I. I seem to be getting intermittent heat from my heater. when the car is moving the incoming air is luke warm to cool, however when I'm stopped at a light, the air will warm up to what should be typical temperatures, only to cool again one I get moving.

I replaced the thermostat 2 months ago, and my collant levels are up to specification. I have ruled out a faulty thermostat for my temperature gauge is constant and does not fluctuate (needle is centered).

Could it be a vacunm or heater core valve problem?

Any help is much needed as we are dipping below zero these days....