I have a stock 04 gtp 3.8SC with 90,000mi. A short while ago I noticed a sputtering/hesitation occuring with light throttle input at low speeds. It is now occcuring at all speeds with light throttle input. As soon as I give it more throttle it runs smooth as can be. The engine light came on and the code read random multiple cylinder misfire. My first instinct was plugs and wires so I installed a set of ACDelco plugs and wires. The engine light went off and for the first day it seem ok but now the engine light is on again and I am still noticing an occasional sputter but no hesitation. I am thinking it needs a new fuel filter and if so where is it located and is it difficult to install.