Open Word docs (in Word) and PDF files (Preview) often repeatedly display the same line or paragraph when I scroll down a page. So I get a whole page or more with just the same two lines repeated. Dragging the affected windows from external to PowerBook display doesn't fix the problem. Hiding the window (Command-H) doesn't help, either. My guess is that it is related to memory or video card limitations ... or it could be the monitor itself. Perhaps other users have this problem as well.
In any case, I found a fix for this issue, albeit after it occurs. If you minimize affected open windows (Command-M or the top-left yellow button) to make them go to the Dock, and then expand them from the Dock, these document windows will display properly. And in my experience, the problem doesn't repeat itself.

