I do not know if this helps or not but I have a 92 W124 that had great power and then suddenly lost significantly - we did the gsxr mod and I removed the ALDA and the plunger is moving properly in response to pressure applied at the air (boost ) port. Fuel supply is good throughout the system up to the injection pump.... at this point I suspect the injection pump itself unless there is internal timing in the injection pump that can be adjusted or repaired.. I do not know if this experience reflects on your situation but as your description sounds very similar I thought I would add my two cents.

best regards