I had this problem on my old iMac G5 but chalked it up to it getting old, but my new MBP is doing it too... It seems iTunes locks up and freezes my computer while playing TV Shows in fullscreen mode, forcing a hard restart or sleep/wake. Sometimes it goes through a playlist of 5+ videos with no problem and sometimes it does it every couple videos. I tried trashing my iTunes/QT prefs, deleting all plug-ins, getting rid of Transmission (heard it could cause problems), but to no avail. Any clue what the culprit could be?