I have a 93 300E and my tranny slipped intermittenly at first as well. I checked fluid levels and they were fine. Finally one day it would not shift into reverse. took it to tranny shop and yes it appears this is how this tranny goes out, intermittenly at first from what I have been told. Had to have mine rebuilt $2200.

Make sure your levels are good! my tranny guy says to keep it at the 1/2 mark as the fluid expands as it gets warm so if you fill to top cold, it will cause your front seal to go out.

My advise .... Start saving for a re-build now!