get a bottle of throttle body cleaner and clean that thing out. It could be because of your MAF sensors that are dirty and the ecu picks up weak signals which messes with the A/F ratio.
the car is supose to cruise on a flat surface at 40-50 mph no problem. if your starting to slow down alot when u let off the gas pedal, u might want to check your alignment or camber.
putting it on is a messy job. I did a retrofit for my g35 and warming it up in the oven to stick it together was a PITA. you will never get as good of a quality as professional work though.