Lines missing in table onscreen

I have Adobe 7.0, Windows 2000, and Word 2000. When I create a table in Word, especially one with some shading, and then create a pdf, some of the lines in the table appear to be missing or incomplete. The documents print fine, however. I've tried thickening the lines in Word and making the shading lighter or darker, with no change in the results. However, if I take out the shading, the lines come back. Is there a way to fix this? Or do I live with the somewhat degraded onscreen image since the printed piece is okay?