NGK Iridiums

[Benzyne]

Hi, I just changed my 92 500SL's plugs from the Bosch ones to NGK Iridiums. It ran sweet as a nut for the test run of 20 miles. After that it sat for a couple of days untill tonight when I took it to the supermarket and it started to miss quite bad once normal running temp was reached.

After searching here I've found that the replacement plugs may have been the wrong choice. I have always run them and found them to be good.

The car has just over 60k miles on it and has been a mint runner since I've owned it.

Whats the difference between a resistor plug and a non resistor plug? And how does this affect the Merc engine?

Cheerz in advance, Tim, N.Z.