It sounds as though a “back emf” from the heater motor is upsetting the rev counter. You would need a storage oscilloscope to isolate the issue. However, it could be worth checking the routing of the wires behind the dashboard to ensure that the heater motor wires are well separated from the power and signal feeds to the tachometer.
On my 4/4, I don’t have that problem but I do find that the rev counter does not zero properly at times - I have got into the habit of always waiting for the whole system to complete initialisation after turning on the ignition before I start the engine.
