This is a known issue in Screenwriter 2000 (4.x) that happens when the mouse is set to scroll "one screen at a time".
To work-around this:
- Click Start > Control Panel.
- Double click on the Mouse icon.
- Click on the Wheel tab.
- Change the setting "One screen at a time" to "The following number of lines at a time:".
- Specify the number of lines that you want the mouse to scroll.
- Click OK.
This problem has been corrected in Screenwriter 6.