It can be a lot of things, is the CEL on?
Pull each fuel injector coupler off (they tend to be a pain in the ass to get off) and check the resistance of each, should be between 1.5 and 2.5 Ohms. Also while its running you can use a mechanics stethoscope to listen to each injector click check out the online service manual listed in the top few sticky forumsso i just installed ngk iridiums and the same issue, so i checked all the coils again and i am sure it has to be one of them, because it doesn't seem to make the engine run horrible like the other ones... so i will buy one on ebay and if that doesn't work, then i have no idea. possibly fuel injector? How would i check those?