ITunes freezes when I play a movie

I just started using ITunes to purchase/rent and playback movies. I purchased one movie and rented another. Neither the rental nor the purchased would play in Itunes. The movie screen for Itunes would freeze and go black when I tried to play the rental and the same would happen with the purchased movie, the only difference being that I could see the play/fast forward/stop bar at the bottom of the screen on the purchased movie. I tried playing both movies on quicktime and I could access the purchased movie but not the rental (i was on a plane w/o internet access so that may have played a factor in the rental access).

How do I get ITunes to stop freezing when playing these movies?