Sudden reduction in high-end cruising speed '84 300D
 

It feels about like it usually has in the lower ranges, but yesterday on the freeway, on the level, when I got up to slightly over 65, it would go no further. Usually, I need to be careful to keep it at 75 - it would jump up to 85 or 90 very easily.

One piece of anxiety, the tach is not working - every now and then I'll still be in second gear, or maybe that's third, my first gear works so rarely I feel like I have a three speed. Anyway, once or twice I'll discover that I'm doing over 60. Holy crap. Probably not good.

Check your switchover valve. Maybe a vac line dropped off.

In theory that is what happens when your fuel filter slowly plugs. You start losing your high-end power. If you went to an interview to be a diesel mechanic that may be the first question they ask. If there is a loss of power what is the most likely suspect and the stock answer is change the fuel filters.

In your case there is the filters, fuel tank screen and the fuel tank vent. Any one or some combination of them can cause that. There is also the fuel supply/lift pump and that fuel pressure relief/overflow valve.

Lots of suspects to contribute to the issue.

If the issue persists after changing the filters get some fuel hose and swap the positions of the Cigar Hose with the fuel hose and that bypasses the fuel tank screen. You need about a 1/2 full tank for that. If that fixes the issue the fuel tank screen is restricted.

If driving without the fuel fill cap fixes the issue you have a fuel tank vent issue. Don't do that with a full tank as some have said fuel comes out while the car is moving.

There is no easy way to hook up a fuel pressure gauge.

Can gunk build up inside of the rear to front fuel lines?

You could get a 2-liter spare fuel can and put that under the hood and run a hose to it. If that improves the high speed, you have some issue with the fuel supply/lift pump or fuel pressure/relief valve. But you have to have already changed the fuel filters.

You can check your turbo boost pressure.