I have a 04 comp g gtp and when i first bought the car it would do 138 mph no problem. I went to zzp and had them tune the car about 2 months after that the car wouldnt go over 120 and it felt like a misfire but never threw a code now on hot days it will do the same thing at about 86 and it will kinda jerk back and forth and not go any faster then 86 somtimes it will do it a 75 mph but it only will do it if i punch the gas the car wont throw any codes i have replaced the fuel filter spark plugs and wires and the air filter the cat was cut off. i replaced all the vacume lines thinking i had a air leak but I have a boost gauge its putting out the same as it always has. the car has 180,000 miles on it but all highway miles abd its well taking care off. I am out of ideas any clues?
