Car:2000 Mustang GT 4.6 liter

mods: k&n ntake, 4.10 gear ratio. and flowmaster exhaust system

problem? When i first start my car (after i let the engine warm up) and i try to redline the car goes all the way to 6k without any problem, but after about 20 minutes of driving if i try to redline the car LAGS alot starting around 4k. Then i turn the car off and back on, it doesnt lag for the next ten minutes but after a while it will start to lag again. I got a oil change a week ago with synth. Everything else seems fine i just cant figure out why it lags, this has been happening since the begginning of summer. The lag comes and goes.Can anyone help me or does anyone know whats wrong with this car?