I am using Microsoft Word 2004 11.2 with Tiger 10.4.5 on a 1GHz PowerBook attached to a second monitor, a Samsung SyncMaster 213T. It turns out that when you scroll in Word on the second monitor, you get this really annoying effect where fragments of the line at the bottom of the page begins multiplying itself when you start scrolling instead of moving to new text.
I finally figured out how to solve this problem after over a year of frustrating scrolling (page up and page down works, just not mouse wheel scrolling or the up/down arrow keys). The trick is that it only happens on the secondary display - and the solution is to switch resolutions of the secondary display to something different than it is and then back to the original.
I don't know how long this has worked, since I just did it, but it is working now. I will follow up to let you know how permanent of a fix this is. I got the idea for this fix from this discussion on the Word for Mac newsgroup.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060324073851133