OVP (Over Voltage Protection) There should be a unit with a fuse in it...depending on your model where it is located. The OVP causes hard starting, rough idle etc. I had the problem once with my 190E hard starting and the OVP was the problem. Another thing to consider would be check the brushes on the alternator, if they are getting bad the output might be falling under 10 volts at idle which might be causing a bad spark. Do a search for "OVP" on this forum and I'm sure you find lots of info in the OVP.
1986 190E 186K (For Sale)
(Maroon, Tan leather)
1996 C280 Sport 208K
(Brilliant Silver, Black Leather)
2002 CLK430 Sport 45K
(Brilliant Silver, Gray Leather , Birdseye Maple)