Could be carbon buildup...went thru this with my 500E (same M119 motor). When you first start your car, your electronics set you up with a rich fuel:air mixture until the car's warm. However, carbon in an engine that hasn't run in a few hours is like a sponge (as represented to me by a BG Products engineer) and absorbs much of this fuel, resulting in a lean mixture. And when the mixture's too lean in a cold engine, the engine stumbles.
I used BG Products' "44K", which is specifically designed to clean combustion chambers. After 2 applications, my cold-engine stumble went away.
Just my experience...and by the way, Techron had no impact