26PF9956/12 After half a hour appears on the right side of the picture a vertical block of 10 cm wide from top to bottom sometimes with stripes.

Take the back off the unit. There are 2 20 pin cables that go to a small board that sits on the right side (viewed from the rear) spray the back of the plugs with contact cleaner and wiggle these cables where they plug into the small board. 

If they are stationary vertical lines in picture and consistently appear after warmng up most likely would be a display issue. displays have control boards attached to them that would sometimes cause intermittent lines to come and go but if when these lines appear, they do not go away without powering off and leaving off for awhile and if no video can be seen behind lines the lcd panel would be where the failure is and would have to be replaced to resolve issue