Smooth-scrolling dynamic text boxes

I have a dynamic text box which loads the text in from an external .txt file. I created an up and a down button which on rollover, scrolls the text up and down. My problem is that the text is scrolled line by line and not smoothly (eg 2 pixels at a time or so). I know that smooth text can be done with static text boxes by masking the text box and moving it up and down behind the mask by a few pixels at a time. But how do I do this with an dynamic text box because it’s length will be changing?

My .swf is here:
The .txt:
The .fla: