Had a less severe problem with my idle, and other misc. weirdness. Got a new OVP for another problem and it smoothed the idle out. What that has to do with new plugs, I think that the guy that sold you the plugs didnt like the way you smiled at the counter lady he is working on getting so he went to the back and got part number 545 548 54 96KM.
the KM stands for "kill mercedes" this special plug sends an electrical gremlin through the plug wires all the way to the OVP and disables 1 small function of the relay thus your problem. Check your box for the KM. Well thats just one thought.