After updating to Mac OS X v10.5.6, computers such as the MacBook Pro (15-inch, Late 2008) with the Nvidia 9600 GT discrete GPU (Higher Performance) may play 1080p content in QuickTime inconsistently. This may be described as “dropped frames” or a “jerkiness” when viewing video.

Link:
Mac OS X v10.5.6: Inconsistent playback of 1080p content when using the 9600 GT GPU (Higher Performance)

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