I had another thought. My '88 300E has a vacuam guage on the dash. It's called an economy guage. It is a manual transmission model, your's is probably an automatic. I don't know if this guage is on an automatic model. If it is, the red area indicates low vacuam. This may help you to check for a vacuam leak as I suggested in my previous reply.

Good Luck,
Larry Bible