Non ho mai capito bene il mc.onLoad, ma mi pare sia diverso dal 'vecchio' onClipEvent(load) di flash5, ti conviene fare così

for(i=1; i<=3; i++){
nome="cubo"+i
_root[nome].mov=false
_root[nome].giu=false
_root[nome].mystop=false
_root[nome].myparent="txt"+substring(_name,5,1);


_root[nome].onMouseMove=function(){
if(this.hitTest(_root._xmouse,_root._ymouse,false) ){
mov=true; _root[myparent].nextFrame()
}else{
mov=false;
}
}
_root[nome].onMouseDown=function(){
if(this.hitTest(_root._xmouse,_root._ymouse,false) ){
click=true;
}else{
click=false;
}
}

_root[nome].onEnterFrame=function(){
if(this.mov){
if(this.giu){
if(this._xscale>10){this._yscale=_xscale-=5;this._rotation+=10;this._y-=5;this._x-=5}
else{this.giu=false}
}
if(!this.giu){
if(this._xscale<50){this._yscale=this._xscale+=5;t his._rotation-=10;this._y+=5;this._x+=5}
else{giu=true}
}
}
if(this.mystop){
if(this._xscale>10){this._yscale=this._xscale-=5;this._rotation+=10;this._y-=5;this._x-=5}
else{this.mystop=false;_root[myparent].prevFrame()}
}
}
}//end For

Non so se ci siano altri possibili problemi, cominciamo provando così