Originally Posted by Govert

Indeed a strange problem. An incorrect vacuum can cause all kinds of shifting weirdness, but I never heard of a transmission slipping out of gear because of it.

First start with the basics: when was the fluid and filter last changed? The fluid level should be below the marks when cold. Best way to check fluid level is when the transmission is warm.

I believe the 1979 300SD has a shift rod to control the shift points and the first gear take-off. Is the linkage adjusted according to specs?

A small investment in a pressure gauge and pressure line should be made so that you can measure the oil pressures inside the transmission. In that way you can correctly set the modulator.

Does the modulator hold vacuum?

You can also use a T piece and a long vacuum line routed to the cabin and connected to a vacuum gauge to measure the vacuum to the transmission during driving.