Poor filtering in the power supply. Sony power is AC converted to DC to a pulse width modulated AC to DC for the set to use. By adjusting the pulse width, the final DC may remain constant as the load varies. That final DC stage may have a bad power filter capacitor.
Mike Andersen
October 2005
this happened with my computer monitor when i first got it. i just hit auto configuration and it fixed it. so im guessing just mess around witht the picture controls