Even though you have restored the voltage on the HD621 by adjustment my first action would be to swap its power supply. Actually that would be my second. My first would be to inspect the HDMI sockets VERY carefully using a magnifying glass. HDMI is not physically very durable and it is all too easy to damage sockets when plugging or even unplugging. My original HD621 was bought ex demo from a Meridian dealer. It kept on giving intermittent problems and went back to M under warranty. They replaced all of the sockets and it gave no further trouble. I asked and they confirmed nothing else was wrong. Bearing in mind you have been moving things around perhaps you have just been unlucky with a plugging action!