It took me a year to solve the same problem with our 124 260E. We did all I thought was right according to the manual, from changing of plugs to tuning but nothing worked till I decided to replace all its injectors.
We had 6 injectors replaced, along with its plugs. That made a fair bit of difference.
The biggest difference to cap of that improvement was replacing a new accelerator cable. Made the acceleration light and the power resumed to normal.
126 tailed by a 203, 129 leading the pack.