Apple has released Final Cut Pro 6.0.6, an update which fixes issues with real-time playback in Final Cut Pro with Mac Pro (Early 2009) models and Xserve (Early 2009) models.
Specifically, Final Cut Pro 6.0.6 improves real-time playback capabilities with Mac Pro (Early 2009) models and Xserve (Early 2009) models when working with complex sequences or high-bandwidth media formats.
Final Cut Pro 6.0.6 is available via Software Update and also as a standalone installer.
More info and download link (19.15MB) here.
[Thanks to MacDailyNews Reader “Jeff P.” for the heads up.]