import mx.transitions.Tween;
import mx.transitions.easing.*;

function slidein() {
var slidex:Tween = new Tween(_root.imagestrip, "_x", Regular.easeInOut, -1000, 0, 2, true);
// when the tween has finished, begin again
slidex.onMotionFinished = function() {
slidein();
}
}

slidein();