TV screen turning really bright and white!! help please

Whenever i turn my sons lcd tv on everything is ok when the sky (via scart) is on, the colour and brightness of the screen is ok but after about 5 mins the screen gets brighter and brighter so it's un watchable.

I've checked the tv brightness settings and they are ok
and if i turn the xbox on this brightness problem doesn't happen at all.

So it must be an issue with the sky box, but what?? has anybody got any ideas?? I'm baffled