Playback Screen Freeze

I'm running a Mac Pro Quad 2.66 GHz with Leopard 10.5.2. When I set to playback in MAIN or Full Screen and issue command F12 the playback freezes (no matter where in the timeline I start the playback) but the audio plays on, and so does the timeline. Only the video freezes. This just started and many playbacks. I don't have any third party cards. Apple stock all the way. I also trashed the plist, to no avail.

Anyone have any ideas?

Thanks, Rio