I am running the ngk iridium in my car now and have over 10k miles on them and they still look very clean with less than minimum tannish color. Why keep spending the money on the standard plugs evry 6 months...thats a waste to me..
you might also want to take some time and check the vaccum hoses and replace any that are cracked/dry rotted and such. take a minute to clean out the little filters on top of the injectors, you never know what might be there, then replace the fuel filter, I have put one of the clear view fuel filters you can get at any parts store in between the injectors and stock fuel filter, just an added security.