You might want to try this. Open QT, go to Preferences > QT Preferences > Advanced and under Video place a check mark next to Safe Mode and then try your videos once again.
Personally, to say that I detest QT would be to put it mildly which also appears to be a widely held view upon reading various forums. Every time there was an update it appeared to mess something else in my system up which is what prompted me to search around the web for matters relating to QT. In the end (and we're going back a while now) I gave up on QT altogether and installed VLC media player which comes highly recommended, is free, and plays back just about every format you throw at it. They do ask for donations (which you are not obliged to do of course) but you might want to consider downloading and installing it and if thoroughly satisfied donating a small sum.
http://www.videolan.org/