sound like a valve miss at certain rpms,most time valves have a high speed miss.Or if a M104 with coils they can do it.As far as grounding a motor,I always add better grounds to engine to chasiss,and one from batt to motor a extra

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1999 w140, quit voting to old, and to old to fight, a god damned veteran, deutschland deutschland uber alles uber alles in der welt