This ought to really confuse the discussion. I have a 78 240D with a four speed. When I bought it from a friend of mine, he warned me that it popped out of fourth, unless you keep your foot on the gas. Which it does.

Thinking about a temporary solution, I decided to turn up the idle adjustment screw whenever I got out on the open road and wanted to go above 50. Sure enough, that stops it from popping out of fourth. Also, using BP premium diesel makes the engine run much smoother, and this also helps.

Looking through the service records, I noticed that the transmission and differential oils have never been changed. I am going to change both today (Redline MTL or Mobil 1 ATF for the trans), and Synthetic 75W-90. I figure this will either make the problem better or worse. I'll let you know.