Ok as title states it i just got my 06 WRX Friday with 106k miles and supposed to be 100% stock. I was on my way home and hit the gas and it pulled great until 3000rpm-4500rpm. It was like it fell on its face then got life and shot off like a rocket again. It runs and drives great the code it is throwing is an o2 sensor so not sure if that can cause that or now im coming over from DSM so this is literally all new to me. I did have a guy around here that knows a little about WRX and STi. He said he belives it was at one time not stock. The clutch is already not stock according to him because it is a pretty stiff strong clutch to push in. He belives it used to have a tune on it possibly a stage 2 and the previous owner attempted to revert it back to 100% stock and the stock tune didnt take well and it messed up my fuel maps. So what i was planning on doing is to get a Cobb tuner for it and tune it and all to see if that will fix it. But i would like to start cheap right now so would plugs and a fuel filter be a good idea to do first or just go strait to the Cobb? Thanks for any help guys. I dont have much time to worth on it i have to report to Ft. Carson in a couple days and dont want to have a jacked up WRX when i get there.