Jhoul.. had very similar symptoms with my '92 400E last eaqrly winter. Based on problems my mother had had with her Nissan in the past, and the fact that I had within the past six months switched to the exact same driving habits as her:

The type of driving is as follows: winter, relatively high humidity, lots of quick (1-3 mile) trips, with a maximum distance on any trip of about 10 miles = Engine never really had the opportunity to heat up.

While I use quality premium fuel, I thought that several months of this type of driving cannot be good, so when at my local Chevron station purchased a couple bottles of techron. I let the fuel run down to about a third of a tank and put one in the tank.

I then made a point of taking the car for a "trip" through some mountain passes when I knew that the engine would be under a good load, and under load at a higher operating temperature.

After some time I did get some pretty good puffs of dark exhaust, when shifting down to passing gear headed uphill, which could be from various sources including the high concentration of techron in my fuel.

Following this, I've made a point of putting some techron in a low tank every couple of months, and the problem has all but evaporated. Certainly worth a try....my $8.42cents worth.....Scott