Took her in
The dealer's mechanic has an M Coupe, and he rode me in it and duplicated what I describe. He was surprised, because the stutter happened in his car, too! We think it is a very little spot in the revs where the variable valve timing shifts.
One has to decelerate with the engine being given just enough throttle to not "engine brake" the car to any firm degree -- which happens for me quite a bit if I'm running behind a car that is slower and want maximum return to power when the chance arrives.
Not a common situation, but odd to put up with.
My car does not do what yours does in the other situation.
Thanks for taking the time to reply, friend.