2006 Altima Blows Cool Air - should be hot
 

I have owned my Nissan Altima for about 1 year. Last Spring when I bought it, I didn't notice an issue. Now, since the heat is now ON again, I find that sometimes, my heat blows cool BUT ONLY WHEN I AM STOPPED. If I am moving, the heat is hot, as I would expect. As soon as I start moving, the heat is there.
I have checked the radiator and the level is where it should be.
Has anyone ever noticed this? Could this be air in the system?