Your problem is that the onEnterFrame function is continuously running. Try this instead:
code:
btn_one._alpha = 0;
btn_two._alpha = 0;
// RollOver
btn_one.onRollOver = function(){
intervalID = setInterval(fadeIn,80,this);
};
btn_two.onRollOver = function(){
intervalID = setInterval(fadeIn,80,this);
};
// RollOut
btn_one.onRollOut = function(){
clearInterval(intervalID);
this._alpha = 0;
};
btn_two.onRollOut = function(){
clearInterval(intervalID);
this._alpha = 0;
};
function fadeIn(clip) {
if (clip._alpha < 100) {
clip._alpha += 5;
} else {
clearInterval(intervalID);
}
}