I'm not a tech, so this is just my opinion.
I would think you have to replace the kick down switch, get the tranny adjusted, or have it checked for any vacuum leaks if it is like the tranny in my 1992 300CE.
Have you had your air filter changed recently as well, if you havent, it might be plugged slightly and causing the car to need to struggle to breathe in air.
Perhaps the accelerator cable needs to be lubricated.
Hope that helps, but I would wait for one of the pro's to answer this, I know quite a bit from reading this forum over the last several months, but I am by no means a pro, or even a tech...
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.