When playing with my PS3, the picture starts off bright and then dims sometimes. I know the tv has 'local dimming,' but I am not sure that is the same thing that is occuring.
Two other things to consider:
1. Your TV may have an energy saving feature that is kicking in. You could try turning off any energy saving features and see if it remedies the problem.
2. Some TV's have an ambient response that changes brightness to compensate for changes in the light in the room, e.g. brightening when the room light is brighter so that the picture shows up better or dimming a bit when the room is darker so that it doesn't appear too bright.