My wife has a 2004 Altima with exactly the same problem ... heater works fine as long as the car is in motion, but as soon as you stop at a red light, it starts blowing cold air. Once you start driving again, the heater starts blowing warm air. It never blows hot!
This seems to be a very common problem for Altima's going back to 2000 and 2001 models, yet I have not found a solution. I sure hope someone here can figure this out as the dealers don't seem to have a clue.