You would need to define the length of your sprite. have that inside another sprite. You wouldn't be able to get line scroll
Setting a mask inside the sprite will define the sprites dimensions of which you could assert an if statement to the top of your action
On frame (1)
if (mysprite.innersprite._y=>end location)
{
stop();
}
On frame(2)
mysprite.innersprite._y=--;
gotoSceneAndPlay("<current scene>",1);
However you can't define an undefined length
Which is why I suggested Brian's tute. The text is dynamic so you can incorporate render as html formation. Meaning inline italic underline bold color fontface and size
If you were to take that a set further you could use CreateTextfield
Which allows a wider variety of htmltext implimentation including images and css.