My 3~4 yr. old 42" Toshiba LCD has what can only be described as a dark "shadow" descending down from the top-middle of the screen(see link below). I thought that perhaps it was dust that was accumulating on the inside of the screen, through the vent slots along the top of the screen. I took a gamble and blew compressed air down through the vent slots for about 2 minutes. This procedure did not produce any noticeable change.
Any thoughts on the cause of the shadow, and any possible remedies?