if the set is very old, 10 years or more ill say its on its way out
if not have you any external loudspeakers near the set either side this can cause discolouration due to the magnets inside them ,move them away if you have
With the set running try switching it off at the mains (not on standby first), and then back on again, this will usually get the degausing loop around the screen to work and should clear the problem. If not then may be the set is on the way out.