My son, his girlfriend and I have 30G ipods - one on a Mac and two on PC's. We occasionally watch videos on the ipods. Although I have never had this problem, they find that, after a period of a few days or so, the videos turn into sound files! If we search for the videos (on the PC, with "Show Hidden Files" turned on), we can find *.MP4 files. These still work in iTunes, but not on the ipod. The files are in the music collection, not the movie collection. The ipod or perhaps iTunes has re-classified them!
If, on the PC in iTunes, we find the hidden files and drag them from the hidden folders into the ipod's video folder, iTunes will crash, BUT the video is revived!