White screen with good audio while watching video on Itunes

I use a TIVO brand DVR. I have successfully converted and loaded over 55 movies from my Tivo to my Ipod. Recently, when I open only SOME of the movies in ITUNES, I get no video, just a white screen and the audio works fine. I had previously tested ALL the movies and ALL worked fine in Itunes until recently. Now I cannot watch the videos on Itunes, but they still work fine on the IPOD. Some still work fine, others not so much.