i.s.d_se3p

first off, hows it going?... well straight to the point, just recently i have been having a really weird problem.... its kinda tough to explain but imma give it my best shot! wen im driving at 4200 rpm in 6th gear going 80mph i start to hear a noise that somewhat sounds like a "sputter".... its a really funky *** noise i have never heard! ive down shifted and the noise just gets louder..it happens every once in a while. well what happens is, my car starts to struggle, the engine sounds like it starts to hesitate, my rpms hardly move and she struggles to pick up speed..it feels like it almost wants to bog out but it doesn't. theres no shaking, no stalling, just hesitation and lack of power... i have already had the cat replaced and new ignition system all done at the dealer... all i got on the car is a k&n intake system... i have come to a few conclusions, maybe im getting to much air and my air/fuel ratio is off. im thinking a vac leak, but i have a normal idle, maybe a clogged fuel filter or faulty fuel pump.... this really stumped me. im fucken clueless.. any suggestions...ive searched the forum and i have found nothin... i need serious responses no fucken smart *** remarks.... please throw some ideas at me. thanks!