It may be reacting to the signal. Is it visible with HD programming, but say, not SD programming?
I've this little annoying problem which at times can get really frustrating... The HDMI Black Level appears & disappears time to time while watching tele... it's a Samsung LCD - LA32S8/720p/HD Ready.
Could this be a technical problem within the LCD? cause if the digital signal wasn't right, it would never appear than. or does it have to anything with the HDMI cable quality?