If you take off the cover and look in the rear of the case (near where the power cord goes in) you will see a small printed circuit board. On this board (which is the power distribution board essentially.) you will see a glass fuse, next to the glass fuse is a black rectangular "box" made of plastic. It is directly next to the rear power outlets. This is the relay responsible for the high power parts of the receiver.
Mine has a similar but different issue. Turns off randomly, then sometimes it also turn on randomly on TV mode. Sometimes when it turns off you either have to bang the side, or wait and it will randomly turn back on. So far my problem appears to be on the front control board that controls the front display and on/off button and all of the buttons on the front of the receiver.
Dave
November 2010