This is a known issue in Screenwriter 2000/4.x, whereby the cursor may jump while typing. This happens when the program paginates and there is a blank element at the top of a page (like a blank character name or action).
Click Format > Run SmartCheck and have the program eliminate any completely blank elements.
This problem has been corrected in Screenwriter 6.
Tags: Screenwriter 2000/4.x, jump cursor