The capacitors in the tach may be getting flakey, causing the erratic needle movement. This wouldn't cause any of the other problems, though.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C