Wow! Some of the users had 3rd party cables while others had the Nintendo branded. The type of TV didnt seem to matter at all. Seems to happen most on very "white " screens which sounds like a sync issue. The topic is currrently 35 pages long!
Nintendo has ignored all the people with this problem and forced them to come up with their own fix using resistors on the video signal! Look at page 15
The only common factor in all of these posts was the Wii itself... not good news.