Heater blowing cold air at idle only

[Mudviper]

I just purchased a 94 Sunbird. When I was driving it around I noticed some thing very strange. As soon as I start moving the car will blow hot air out of the heater, but as soon as I stop it would start blowing cold air. I start to move again and the hot air comes back. Is this normal for a Sunbird. If not, what would cause something like this. I know if it was just blowing cold air out all the time what to do, but have never seen this happen in any car I have owned

Thanks

 

[DanSavage]

If this is a 2.0 Litre engine, your timing belt is on the way out. It drives the water pump.

 

[melsg5]

does the temperature gauge move when this happens, low reading when cold air, higher reading when warm air?