I also have an '89 300se and after three years have most of the bugs fixed. The 6 cylinder engine seems very reliable and not as thirsty as the MB V-8s. Problems include, leaking timing covers, and oil consumption from tired valve seals,

Your trans slippage might be from a vacuum modulator which has a perforated diaphragm. Mine did this and the engine drank the ATF like it was fine wine. When the fluid gets low you'll have that slippage.

BTW, do your seat memories work?