That's Odd

[Ren Dao]

It seems if I put the heater on or open a window the speedo stops working ..... until the car has done a few miles - any ideas !

[GreenV8S]

I suspect the voltage on the ignition switched circuit is dipping, on mine this takes the tacho with it. Does the voltmeter reading drop at the same time? Potential causes would be poor contact on the ignition switch multiway plug (a common favorite), ignition switch itself starting to give up the ghost, or a voltage drop across the immobiliser.