Z3 Heater not working properly

[b_OASIS]

The heater in my 98 Z3 doesn't want to work properly. When I turn it on in the morning to warm up, why the engine warms too, it blows nothing but cold air. The when the car is in motion, hot air begans to come out.

Ok, now here is the kicker, cause I know what you're thinking. When I stop at a red light and the car sits idle, no longer moving but the engine's running (and the car is still in drive) the car once again begins to blow out cold air. And by now the engine is plenty warmed up. used:

My question here is, is this an easy and inexpensive fix or am I looking at a serious problem. And if anyone knows what the problem is, feel free to diagnose it too.

[Ralph85735i]

I'd guess a couple things to look at:
a) is the controller fully opening the butterfly controlling hot water flow to the heater.
b) is air trapped in the heater core, allowing only partial flow
c) warm air at higher rpm's would be consistent with either a or b

Feel the hose feeding the inlet to the heater core. Is it warm/hot?