I have the same problem on my 190e. Has been there for 10 months. Have spent nearly 2K. Still no change. Two mercedes dealers and two independent mercedes techs have not been able to figure it out. have changed plugs, wires, cap/rotor, injectors, OVP relay twice (everyone seems to blame everything on the OVP relay, but it doesn't always help!!), air/fuel filter, thermostat, installed a 470 ohm resistor on cis-e circuit, reset timing, NOTHING WORKED. These are all the standard things everyone replaces, without even checking if they need replacement!
This is a fuel system problem, but the dealers would rather throw parts at you than diagnose the real problem. If you solve it, do let me know!