The 1980 300D N/A did have a vacuum-only controlled transmission.

The vacuum controlls both when the transmission shifts and how it shifts.

Also 1980 transmission had a feature not found on earlier N/A diesels: first gear take-off. When it stands in idle, it is in second gear, but when you press the accelerator pedal it shifts back to first gear. This is also controlled by vacuum.

On top of the valve cover there is a switchover valve, also known as 3/2 valve. At idle, it provides high vacuum (22 inHg) to the transmission. When you press the accelerator pedal, the valve switches to the lower vacuum from the Vacuum Control Valve (VCV). The VCV starts out with about 10 inHg of vacuum and it further reduces it to 0 when you fully depress the accelerator pedal. The sudden switch from high to low vacuum causes the shift back to first gear.

The switchover valve (or the little flipper that actuates it) wears out over time, so that the valve leaks. This will cause 0 vacuum and the transmission "thinks" you are pressing the accelerator pedal fully for a hard acceleration and therefore late (and hard) shifts.

So a check of the vacuum valves would be the first step.

It could also be something more serious, but vacuum is more likely.

If the transmission oil and filter wasn't changed recently (within 35,000 miles), you should do that too.