1993 BMW 325is heater

1993 BMW 325is
my heater only works when the car is moving and works best on the highway. i have brought it to my mechanic 3 times or so now and he fixes it each time but then after about 2 weeks it fail again and goes back to the same problem; blowing out cool air when the car is on but idle, and when i move it warms up. even if i am moving a great deal say 20 min on highway it gets nice and warm but once i get off the highway and come to red light or stop somewhere it goes back to cool air. i got a new radiator, and still have problem. my thermostat does move sometime; it will move towards overheating and occasionaly light red but most of the time it will quickly or steadily go back to the center before it overheats. anyone know what the problem could be or what should i do to finally fix this especially since i brought it to my mechanic 3 times. my family has been going to him for years and he usually knows his stuff