Check the ground connections behind the instrument panel. There are 2 clusters of brown wires connected to 2 ground points on the steering shaft. If you pop the instrument cluster forward and push the speedo cable from inside the engine compartment you should have enough room to see and get a long extension for the socket.

Mine were loose on the 240D. The gauges were jumpy and they also flickered when I used the turn signals. Once the ground clusters were tightened, solid as a rock.

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79 240D (no name yet) 203K Miles Maple Yellow
98 2500 4x4 HD Dodge 24 Valve Cummins TD
83 Chevy Van (Dead-Trans. shot)