George and Malcolm,
The problem occurs on all speakers indeed. I did forget to mention that in my earlier reply. I will follow George's idea and start with a small system. In my opinion it is a problem with the G68. I will report again.
Paul

G68ADV, G98DH, 7000, 5200, 5200HC, JVC LCD, Samsung HD cable-receiver, Gefen 4x2 HDMI switcher, MIT cables and Logitech Harmony 1000.