Trash the preferences with this http://www.digitalrebellion.com/prefman/
If things don't improve take a look at this troubleshooting article
I'm having similar symptoms:
I've bundled a series of multicam (2 camera) interview questions into a compound clip. The compound clip itself is now showing black where the thumbnails should be. When I skim the timeline, the preview window sometimes shows me the footage and sometimes it's just black. Sometimes actual thumbnails will start to show up in the timeline, but then they disappear again. While skimming, audio generally seems to play correctly. And other video on the timeline works fine.
The only thing I can think is that my hard drive might be too slow (I'm using an external USB 2.0...silly slow hard drive to use with my new iMac Core i7, I know). But if it's the hard drive speed that's the issue, it would be nice to get some kind of error message. Honestly, it was working just an hour ago, so I really don't think it's the hard drive.
I've restarted the computer & the app. Trashed preferences. Hidden projects I'm not using. Booted in Safe Mode. Repaired disk permissions.