Just bought a graded Sony 36FS70 and find there are dark shaded blotches n all 4 corners, when a white or light coloured backgrouns is displayed. Not visible when darker pictures are on screen. They are not linear as would be the case if it had been left in 4:3 mode with that type of screen burn. Is this possibly a setup problem, or is this maybe why the set is graded and sold refurbished? Any thoughts.