This may be caused by the TV's Eco Solutions features which will automatically dim the TV depending on the brightness of the room. Try adjusting these settings (Menu->Picture->Eco Solutions) and see if that alleviates the issue. The main feature you will want to adjust will be the "Eco Sensor" setting.
Samsung LED model UN55C6500VF. While watching TV (any channel) the picture suddenly darkened. After a few minutes it gets lighter again, after another few minutes darkens again. This continued throughout the evening. The tv is only a month old. This is the first time this has happened.