I read about this problem the other day, and it is something to do with the record mode or rercord standard you are using. So NTSC is 29.97 frames per second, and your TV tries to scan at that rate, but if you recorded in PAL which is 30 fps, then you get black lines, and vice versa i imagine from PAL tv with recorded NTSC settings.
Also could have something to do with interlacing the video during conversion. May be a setting for de-interlace that needs to be selected/un-selected.
if you can't find the answer here....search on forums.macrumors.com in the digital video section.
Best,
Brain