I had problems with resistor plugs. The auto parts store where I was buying plugs sold me plugs with an "R" in the number, stating they were what the manufacturer listed in their book as the replacement plug for my E320. So I took them and installed, not knowing any better. They ran fine for about 500-800 miles, but then I developed an unusual stumbling at idle. Felt like I was getting bumped in the rear. Replaced the plugs a few times with different brands, all with "R" in the number, with the same outcome. Searched all the BBs I could find and never got anywhere. Had people telling me to replace the OVP relay, MAF sensor, plug wires, ignition coils, ad infinitum.
Finally I had an epiphany after doing like you and researching plugs on the NGK website. I learned that the "R" in the plug number was for resistance. I surmised from kitchen physics in college that the by-product of resistance is heat and that likely thinks were a little too hot down in the old spark plug holes. I also had read about the resistor ends on the spark plug wires that attach to the plugs. With the resistor ends AND the resistor plugs, too much resistance. So I ordered a new set of resistor ends from performance Motorcars (about $70) and went to the dealer, which had THE CORRECT DAMN PLUG as specified in the owner's manual. Installed all in about 30 minutes and to date (about 6 months) no more stumbling idle. Moral of the story - Use the right plug in your car. Forget the platinum, splitfire, resistor, double trouble hoopla and get the good old copper plug specified in the cars literature.