Watching TV with sound via Denon AVRX3300W the tv will randomly switch to TV internal speaker! I have changed the HDMI cable, done a power off and a factory reset on both TV and AVR. Watching a film from a media center or watching a youview chanel via the AVR there are no problems. This random switching occurs only watching the TV. Has anyone any idea what might cause this as it is driving my wife and I mad. I would be great-full for any suggestions.