On the 124s the heater valve is electrically controled thru the pushbutton assembly. The 12v signal is constant but the ground is the controler. The valve needs both signals to close off the flow of hot water in the heater core. So you need to know if your power and ground signals are both present at the time of the problem. To check this unplug the heater valve element (located next to the battery, the brown valve with heater hoses going to it) and put the negative probe of your meter in one socket and the positive probe in the other and the result should be battery voltage. If battery voltage is not attained then you have a control problem or wiring problem. If battery voltage is present then the problem would likely be in the heater valve itself.
Hope this helps,
14 year MB tech
7 year shop foreman @ independent MB/Volvo shop