Why does my Toshiba tv screen go from dark to light?

If you go into picture settings and change but same bad board or screen

The TV's main culprits to be causing this problem are "dynamic contrast" which maybe darkening the backl light in dark scenes an example a night time or a dark scene in a movie. To make the blacks be deeper (as LCD/LED TV are rubbish at blacks) or brighten it on bright scenes to make the light be lighter and once again to make the blacks be deeper so the images pop out. Also "eco lighting" this will dim or brighten the image based on the amount of light in a room in order to be saving energy. Ether way the easy fix is to turn both of these features off. You will get rid of the problem and enjoy more subtler shades of colour that would otherwise be bleached (brightend) or crushed(darkend) out..