do a search on "flaring". technically, it's not really slipping but the engine revs up before engaging the next gear. i believe there are adjustments you can perform on the vacuum modulator. look for Steve B.'s posts on this, you'll probably find out all there is to know...
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"