Just wanted to give an update. The car is an '86 300E 135K Miles.

Changed the fluid and filter. The old stuff didn't look bad and didn't smell burnt. Very little debris on the bottom of the pan. However, no help - tranny still doesn't like to stay in 4th gear.

Then I tried adjusting the pressure valve. I bought the factory manuals ($140) and guess what - tells everything about the car except for transmissions. Read some old posts and started turning that little "T" in the pressure valve behind the nickel size rubber plug (is the plug the only thing that keeps it fromfalling out?). With one turn it was working better. Two better yet. Now I'm at four turns and it's pretty good. 1-2 is harder than before but 3-4 shift is not quite as good.

Total cost $30. Total time 3hrs (with test drives between adjustments). Outside temp 105° (I would do this on the hottest day of the year). I'm probably running on borrowed time with this tranny but I'll take every minute. Thanks to every one who replied.

John H