The 1970 280SL has a vacuum switch over valve. Vacuum is supposed to go to the distributor up to about 2,2000 RPM and then shut off. Check that and see if it works.
If you have a fuel problem it will slow down quickly and loose power. If you have a timing problem it will tend to hit a certain speed and not want to rev up any higher. Ignition problems tend to cause rough running with spark plugs more of a miss.
The injection pump will have very eratic running over a wide RPM range. The engine may idle smoothly yet start to sputter as you increase engine speed with the car in neutral. No amount of tuning will change how it runs. After testing fuel volume, plugs, valve timing, ignition wires, points, distributor advance/timing and you find nothing wrong it points more to IP trouble.
As a major part of the car and specifically the engine, the rebuild costs for an IP are not as high as some make them out to be. IP rebuild is less than having the head pulled for a valve job. It's definitely less than an engine rebuild, more than a rad recore, more than the price for a soft top but not installed, depending on where you buy a fuel pump it can be more or about the same price, less than a paint job, less than chrome plating, more than a carpet kit without installation and about the same price as a full set of re pro bumpers.
However, aside from the mechanical things I just listed, it's one of those things you NEED to make your car run. Which makes me wonder why I see all this wailling and gnashing of teeth every time someone is confronted with that prospect. It's a fuel injected car and it has a IP. Try pricing a new fuel distributor or an ECU for a late model MB and you'll think your old car looks better every day.
Anyway I got a bit off topic and you have some things to look at. Let us know what you find.
