When my brother had the 1985 190e, we had a loss of power, you could rev it up to 4k in neutral, but if you put it in gear, the revs would stay very low, and you would not be able to get up to speed.
The cat was plugged, and needed to eb replaced.
After replacing the cat, the car was fine, until one day the tranny just up and lost 2nd,3rd,and 4th altogether.
My brother had not taken care of the car, once he had driven from sonoma, CA to Valencia CA on one tank of gas, with barely any oil in the car.
When I met up with him in valencia, we ened up having to add 5 quarts of oil.
But he did get great gas mileage on that trip... LOL
Needless to say, I would htink have the exhaust system checked like others said, have the tranny checked out to make sure its ok, and if all else fails, have the lifters checked.
My brother's 85 380SL that he has right now, just had the lifters done, and he says it has much more power than before he had the lifters done. I believe its the same engine as in your car but I am not sure if yours has the dual timing chains like my brothers car has.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.