If you have a connection to the camera using standard RCA barrel jacks, there may be some oxidation that is causing lack of signal. As motion ensues, the connector will seat and pass signal. The cure is to treat the connectors with an antioxidant spray or liquid like Caig Deoxit.
Another problem may be the location of the wires that connect to the camera. If the wires are exposed to the road surface, a pebble or small rock can impact a wire and part it within the jacket. You will not be able to see it. This happened to me with a wire to my trailer socket. The wire lowest to the road was hit and there was an intermittent connection to the trailer brake wiring. I discovered the break in the wire by feeling the indent in the wire jacket.