Well my 03 gt just don't seem to have the power it should. I've had it 6 months and changed oil, fuel filter, plugs, and cleaned the t body. Also cleaned maf, egr, and iac. First test drive she broke loose in 3rd (it was cold and damp outside) now it maybe chirps in second. Sometimes going 45 in 5th if I hit a slight incline it gets this chug/bog, and u have to give it alot more gas or downshift.rarely though when its running better( this seems to be intermittent but most of the time runs bad) the same hill at the same speed it has no bog and gas pedal barely has to move. Also exhaust note varies slightly when it's running better/worse and sometimes pops on decell/shift more than others. Mpg seems a constant 20. I have no codes except for downstream o2 as I have or x. No misfires or anything drastic, it still runs smooth, and has power, but definitely not what it should have. I'm thinking maybe a sensor or something. Also i replaced the intake due to small coolant leak soon after i bought it. It is hard to tell if it lost power after this as i didnt really drive the car hard for a few months. If there was a tiny air leak or something would it not set a code? The car does have 160000 on it but the insides are very clean as the previous owner puts many hwy miles and maintained it. The reason I know it's not running 100% is because there have been days here and there when it pulled hard and you knew it was right lol. Please help I'm running out of ideas.