One other thing to think about is the aux water pump. The heater valve is pulse width modulated. This means that it pulses more or less to control water flow. On 124 cars the heater core cools off at idle due to lack of flow. The aux pump needs to move the coolant in these times. If it doesn't the valve is pulsed further and further open and then when the car leaves the light the rush of natural circulation gives a burst of heat before the valve can compensate.
Took me a while to find this the first time. It won't show up at road speed.
Bosch Master, ASE Master, L1
33 years MB technician