I am having the same issue on my 420sel. My mechanic says it could be the cold start injector.

I just had all 8 of my regular fuel injectors cleaned and tested and they are all working correctly.

Also try this...

my car seemed to fire fine this morning after i forgot to attach these connectors after replacing some things.
If you look at the top of the radiator and follow the big pipe, theres 2 or 3 electrical connectors attached to the metal piece at the end of that coolant pipe - i believe they are temperature sensors. Disconnect that and try to do a cold start. Does it start better?