I have a 5g video ipod. Sometimes the movie plays in a tiny window on the side of the title and time. When this happens it is hard to correct it and I'm not sure why it happens or how to fix it.
I am clicking from movies. It has always had this problem. The firmware was updated several months ago. Today I selected a movie with the center button and it worked fine. Later I forgot and selected a movie with the play button and it worked fine too, over and over. Most of my movies are handbrake encoded from dvd.
You might want to try restoring your ipod from iTunes, it will reinstall the firmware and then copy everything back over. Maybe something got corrupted.