Did you try to wiggle tach amp while needle is bouncing ? Also wiggle cluster connectors (space be tight with oil line connected). If you can get the bounce to stop while wiggling you found the problem area.

Also measure the voltage supply to the cluster, it should be steady and not fluctuate with the turn signal.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked