The tv is a new lcd screen and the wii is also new.
The tv has one gold plated scart connected to a 3 way scart adapter. The adapter has a dvd player attatched, a sky satttellite box attatched and the nintendo wii via a video/phono lead to scart adapter.
The change in brightness is intermittant and only occurs with the wii, not sky or dvd player.

This may be an adjustment in the settings on the Wii and not the tv. Try going into the system settings and adjusting it.