I have a similar problem with my 280SL project which I am finally able to drive. It has an injection pump that was supposedly rebuilt in 1997. Fine until it hits operating temperature, but then it will mis-fire under moderate load like normal acceleration or climbing a hill. And it takes more throttle than it should to get it to accelerate or hold a steady speed. The diagnosis was a problem with the injection pump, or an incorrect partial load adjustment.
The suggestion is to 1) send the pump to a specialist for diagnosis and repair, or 2) try to adjust the partial load setting with the pump on the car and see if it runs better. I tried this, went 1 click richer on the adjustment, and it seemed to help a little. A problem is that a fair amount of oil runs out when you take the cover off to make the adjustment, and you need to use a mirror to see the screws.
Not sure if this describes your problem.
