How to fix my integrated LG Tv and Skybox?

Once switched on from cold the TV will operate without any problem, all night. However if it is turned off and on again, whether cold or hot, a black square will appear in the middle of the screen blanking most of the picture. If it is turned off and left alone for half an hour or so, it will operate normally.This happens regularly without fail every time it is turned off and on again

Emjay
March 2010