"Random Movement" within a fixed space

Hi All:

I’m using the Kirupa “Random Movement in Flash MX” tutorial and by taking out all references to rise and x I’ve gotten the effect I want with horizontal lines just moving up and down randomly.

I would like to know how to define the space in which they move however. In the tutorial, I guess it presumes that you want the object moving throughout the entire space of your movie, but I only want them moving within the space between the 60th and 80th pixel vertically, meaning the line should only move up or down 20 pixels at the most.

Can anyone help me with this problem? Let me know if I’m not explaining it clearly enough.