I have found that just switching the plugs is not enough..if the car has been running with incorrect plugs over any extended period...you will likely need to go back and change the dist. cap. rotor and plug wires, as well as the plugs...even if all those items were new at the time of installing the incorrect plugs. The problems you are seeing are not necessarily from a deterioration of the incorrect plugs themselves, but from the damage they seem to effect on the whole ignition system. The effects seem cumulative..and since you said the car ran corrrectly for 12,000 miles before showing symptoms, I suggest you follow the procedure. Try to use "OEM" for everything else other than the plugs unless they are the correct copper non R versions.

I solved multiple issues on my 2.6 (w201) including the hard starting, idle shudder, periodic runaway idle (up to 3000 rpms occasionally), poor gas mileage, stalling when coming back to idle, simply by executing the procedure above. These symptoms did not all happen at once but progressivly got worse and more extensive over time. The change was as dramatic as flipping on a light switch.

I say use MB as a source, because aftermarket Bosch stuff comes from all kinds of different sources..which are not necessarily equal. I cannot tell you how many different versions of what should be the same parts I have recieved as after market supplied "OEM" Bosch.