Can you tell us about your source footage, and maybe also post a screen-cap of this behavior, showing both the Timeline and the Program Monitor?
Also, please post details of your computer, especially the video card, and the date of its driver.
That would be helpful.
This is aimed at Premiere Pro, but may help
A link with many ideas about computer setup http://forums.adobe.com/thread/436215?tstart=0
Many of the ideas at the next link are generic enough they have helped some Non-Premiere users fix problems
Work through all of the steps (ideas) listed at http://ppro.wikia.com/wiki/Troubleshooting
If your problem isn't fixed after you follow all of the steps, report back with the DETAILS asked for in the questions at the end of the troubleshooting link