The problem you are seeing/feeling seems to be very common in the 1992 400e. I have the same problem along with a few other 400e MercedesShop.com members.
Do a search on "400e transmission" and I beleive you will get a few hits. If I remember correctly, it is a design issue with the B-2 piston.
My only advice on what to do with the power..... ENJOY IT!