Check HDMI port on TV very closely - it might be some dirt in the connector itself - use a toothpick or very little brush and try to clean it (compressed air helps as well).
Next step is trying with new HDMI cable (some of them are even device sensitive - check picture on connector computer/TV out)
Check HDMI connector on computer side as well
Usually this kind of trouble happened when cable connector is not deep enough in HDMI socket.