It's probably because you're using a U.K(PAL, 50Hz) Wii on a U.S (NTSC, 60Hz) TV and the TV doesn't have support for PAL 50Hz.
I do believe you can region change your Wii (requires softmodding) to fix this.
However, I only own CRT TVs, so this may be because it's a plasma TV.
Have you tried enabling progressive scan in case your TV uses progressive display? (Wii Settings --> System --> Resolution)
(If your TV uses interlace the screen will go black. To change the Wii back to interlace, turn off the Wii and unplug the A/V cable, then turn it on again.)