I had these symptoms (would not want to accelerate past 45 mph, but playing with the throttle would help a little but if I kept the throttle to the floor, then it would, after 4 or 5 seconds, accelerate which was not ideal if one wants to overtake).
It proved to be the Mass Air FLow sensor. In my case, it was simply dirty because I had used the car for a little while without an air filter.
Cleaned the MAF sensor with artist paintbrush and alcohol and all returned to normal.