Sounds like a tricky one. Need someone who knows how to diagnose a car and not simply "replace parts". The shotgun approach can be quite effective, but MAN that ammo is expensive!
I am not suggesting the car can be repaired over the net, either. Need that personal touch sometimes.
It does sound a little like a symptom I have seen though on 190's. Does it act like this while driving as far as a difference between "normal" acceleration (bogs out) and if you really "baby" the throttle? What I have seen in cases like this is a plugged catalytic converter or muffler. No damage (kinks) in the exhaust system? One cheap way of testing this is to loosen the exhaust system from the exhaust manifold and try driving it then. Just loose enough so you get maybe 1/4" clearance between the front pipe and exhaust manifold.
On the 2.3, be very aware of the possibility of a blown head gasket. Look for oil in the coolant tank, that's usually the first sign.
Has the on-off ratio (lambda) been correctly set? CORRECTLY is the key word there.
Make sure the correct spark plugs have been used, I don't care what the parts-replacers say about the scope.
Please no more "PPPLLLLLEEEAAASSSEEE!!!" type whining, very unbecoming to a Mercedes owner, even a 190 owner (oh, did I say that??)
ps I have seen with the plugged cat problem that top speed will also be affected. I have seen them top out at only 50 to 60 mph, so be aware of this. I believe your 85 has the 2 cats on the very front of the front pipe (can be seen fron under the hood) these are prone to plugging.