I think you need to be a little more specific for people because there really is no apparent problem that you state. Maybe some video?
You are going to feel a big increase in speed to 30-40m because that is the main part where you are accelerating. You are probably hitting max v 40-50m or so, if not sooner, so you simply are not able to accelerate any further (if your car can only go up to 100mph, it won't go 150mph just because you push harder on the pedal).