i have a slight problem with my 05. the only mod is a k&n short ram, and the 3/16 mod - around a month ago the car would surge in third near 4500 to 5+ rpm - like boost spikes, i think as i can hear the wastegate.
then it went away the same day
then today the car did the same, but in all gears and it had no power near and past 5k - i tried to wind it up in 3rd ( near 5k ) then shifted to 4th and it popped from the intake - not a backfire, but a pop and still no power
i i lean into it slow it is ok untill 45-5k
i have heard that the EGT sensor can cause this. no engine light and no codes