Kerry is right on this. First gear was a 3.68. So you probably are just asking too much of that engine at low rpm. Here is a great site (though somewhat hard to navigate) with lots of the gearing info about our cars:
For your car:
W126-300SD-Getriebe/en

For others, navigation is best started here:
Hauptseite/en

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

Wish I had known that before starting. I could have easily got a 4 speed tranny. O well. I think switching to 3.73 gears will def help. I have an egt in the mail right now, once I get that, I am just going to remove the alda all together and see what the effects are.

What is the condition of your wastegate rod tension? If your wastegate rod tension is correct and your timing is correct and your fuel tank is not blocked and your filters are clean and you still don’t have the power you want; then it is time to start turning those screws in the back of your injection pump like they do in Finland or Sweden or time to have someone else to do it for you. Those engines are strong, as I had one to tow another 300SD almost 500 miles to include pulling up mountains. So, I know these engines can turn your big tires without issue, once you get it figured out.

BenzDiesel