Why does car shake violently while RPM needle decline on cold start?

What is your question?

Car shakes violently until RPM needle reaches lower level from 1000 to 700 on cold start. Needle does not fluctuate back and forth during decline, just declines in increments in one direction. Before repair of worn engine gaskets RPM needle remained steady at 1000.

What makes this problem better or worse?

More severe vibrations on cold start, does not always happen again after first start. Not too noticeable if car parked with front of car in decline

How long has your 2009 BMW 528i had this problem?

Since after mechanic took engine apart to replace coil pack and replace worn engine gaskets and seals 02/2018