LCD screen shading marks

[g*****s]

Have read alot about these shading marks,sometimes they look like clouds,sometimes more like patches or bars.But usually can be seen only on some picture states.But if you want to check for them put your set to blue screen mode,ie with blue screen turned on in menu and a non tuned in channel selected and if they are present will show up.Have read they are caused by heat affecting the diffusers inbetween backlight and screen.But having changed seid diffusers,me thinking more allong lines of screen fault.That is unless anyone else knows differen.