My 300se had begun running very rough...since I've just purchased the car my guess was a cap, rotor and set of wires and plugs wouldn't hurt...little did I know that it's $400.00 worth of parts. When it didn't improve the missing didn't get better I was stumped. A close examination of the injection harness proved to be the culprit. The coatings on the wiring harness were disintergrading. I'm in the process replacing the harness...$700+.