No compressor cut-off during full throttle accel except on diesel MBs.
Unfortunately your problem is pretty strange for an 124 car. It does not have a vacuum controlled heater valve as in the 201 cars. If you are loosing vacuum the only way this can affect heat is for the recirc door to go full open (something not normally possible in A/C position). If this happens the center ducts should close and the air should go through the defrost ducts. And this is only going to give heat if you are already mixing heat at the temp setting you are running. In other words if ypou are in the MIN position the heater valve is dead closed and mixing hot outside air will only make the air warmer but not heated. If on the other hand the heater is working because you are set at 78 and there is mixing to achieve a temp greater than A/C (45 deg), then when trhe doors move heat actually can occur.
Another way this happens is while asking for mixed air the controller can't achieve a satisfactory heater core temp while idling do to the aux water pump not working. As a result while sitting at a light the control duty cycle of the heater valve is increased and increased (because at idle no water is flowing). Now start of from the light and the core is flooded with hot water due to the open state of the valve. This usually is only a winter condition in Florida (my experience), but could easily occur in the evening with large mixing. To check run in the MIN position and this won't happen. Also check by pinching off the heater hose.
The doors will move if vacuum is removed but they dont directly affect heating and are easily identifiable as the door position will quickly change.
Bosch Master, ASE Master, L1
33 years MB technician