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From your description it sounds like suddenly the video will stop after a few minutes, but the audio continues as normal.
I would start here with this article to check for and troubleshoot 3rd party plugins.
iTunes: Troubleshooting issues with third-party iTunes plug-ins
Also there is another article that may help with video playback issues in iTunes named Troubleshooting iTunes for Windows Vista or Windows 7 video playback performance issues found here http://support.apple.com/kb/TS1718.
And if the issue persists after all that, it may be a good idea to uninstall iTunes and its associated applications, then re install them. This article will help you with that.
Removing and reinstalling iTunes, QuickTime, and other software components for Windows Vista or Windows 7
All the best,