Grid heater or glow plugs in that motor?
Seems the speedometer is getting an electrical pulse from the engine heater going on and off. Is that preheat light close to the speedometer, might be some dust/moisture connecting them on the back of the board. Then when the heater is off the issue goes away, which is a good thing to watch. Wouldn't worry about it till it works otherwise.
My shot in the dark, lol