How to ask a question http://forums.adobe.com/thread/416679
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
And even more hardware discussion http://forums.adobe.com/community/premiere/hardware_forum
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
Also, remember to render your video if it has a red line above it on the timeline.
To render your video, press Enter on your keyboard. The red line will turn green and your playback will show a high quality preview of your video.