DVD video has horizontal line travel
 

I'm sure this has already been posted, but I am at a school for video at this very moment and we need an answer as soon as possible. Every Friday we put on a multicamera news program with commercials and PSAs, but all the DVDs burnt on our computers (new iMacs) the video has visible horizontal lines that travel upward on the screen and we can't figue out how to change it. Our teacher has us use iDVD to burn the commercials and I though it might be a scanning problem or a format difference between interlaced and progressive, but apparently it use to work and we haven't changed anything. Oh, and it's just the video after it's put on the dvd that's affected; when it's on the computer it looks fine. Any ideas would be great since we go live at 12:30.