What are your temperatures like?

I bought an '84 300D recently and it too wouldn't get past 65mph. I did the fuel filters and linkages which made no difference.

The temperature was running low (under 80 deg C even at freeway speeds for 30+ miles). I replaced the thermostat which was a rusted mess and the temperature went back to normal and I was able to get it to 75-80 mph. After re-connecting and adjusting the ALDA, I can now easily get it up to 90mph and have had it just shy of 100 on a few occasions.

Can you feel the turbo kicking in at lower speeds WOT? Is the acceleration normal for city driving? Before I re-did my ALDA, I couldn't even tell that I had a turbo diesel. If your boost pressure is way down, you may have trouble with higher speeds.

Another thing you might want to check is your accelerator pivot mount. The rubber gets bad over time and when it starts to split, it still works for a period of time until it leaves you stranded somewhere. Before it fails, you will have some strange acceleration-related issues.

Dkr.