Its because the wii outputs at 50hz rather than 60hz for NES, SNES and Megadrive games.
I have emailed Codestorm about this who are almost useless and take days to respond. They initially stated only a few tv's are affected, but when I asked them to supply me some lcd tv's which were not affected they were not able to. They were not able to give me any info on the problem, except Nintendo were looking at it, and once its resolved nintendo will issue a press release.
Hopefully Nintendo will sort this out and my Wii purchasing is on hold. Just in case I have to purchase a US version, and replace the 8 wii games and 16 VC titles I have already purchased.