Ford Explorer 98 4*2 does not revolutionize when you put veloc

Hello, first of all, greetings to all of the forum, my problem is that my Ford Explorer 98 truck takes a long time to turn on the engine march is unstable but it remains on when the time comes when it responds very well when accelerating but at the time of putting speed stops revolutionizing and has no strength to move forward is a truck that is newly adjusted.