I drove it today for several miles and the first good news is that the incredible shaking at idle is totally gone. There is a quiver at acceleration, but I think that may have been there before and may be due to the high mileage as the tech mentioned.

My 300 has the same mileage and it is as smooth as ice. The 420 is still pleasurable to drive, it is just not as smooth as the 300. I guess I have to finally come to terms that one day I would have to deal with the engine's high mileage. It turned 236k, but still runs very strong for a 15 year old machine.

Overall after a 1 month fiasco and being in the shop for 2 weeks, I finally got the car back!!