I have the same problem with my 1989 190E 2.6 - here is my story. Initally I had the cold start problem, after changing the OVP that went away but this new issue is now occuring from time to time.
When the car warms up I would dirve it for about 1 - 2 Km and al of the sudden when I try to accelarte the engine would like cut off and the car won't speed up but rather slow down, as soon as I go to a complete stop I can barly move forward it will go bu tabout 2 km/hour. When I shut the car off for about 10 minutes start it up again and it would go fine and get me to my destination.
I did the general tune up (spark plugs, cables and dist cap, fuel system cleaning and carb cleaning).
After reading many of the posts here I am aware that there could be a problem with the fuel relay or the o2 sensor - or do you guys have any other ideas?