I've got the same problem in my 87 300E. I notice that it only happens after the engine has warmed up. I've replaced: Cap, rotor, plugs, wires, injectors, OVP relay. (no luck.)

I spoke to a mechanic recently, he suggested 2 things. First, disconnect the O2 sensor, and see if it still happens, if that fixes the problem, get a new O2 sensor. the other thing he suggested was to remove the air cleaner, and locate the "EHA Valve" and see if there is any fuel leaking. I forget what EHA stands for, but it has to do with the fuel flow or distribution.

Don't know if any of this helps, I hope to work on this issue this week, if one of these works, I'll let y'all know. This seems to be a common problem, and very frusterating to isolate.