Changing alpha

onEnterFrame = function () {
box_mc._alpha -= 5;
if (box_mc._alpha == 0) {
box_mc._alpha += 5;
}
}
HERE IS A SIMPLE CODE THAT I HAVE ON THE MAIN TIMELINE, THE ALPHA GOES DOES DOWN BUT IT DOES NOT INCREASE WHEN THE THE ALPHA HITS 0. WHAT IS WRONG WITH THIS CODE.