Giving this a bit more thought when I switch on the picture and sound come on ,and then for a brief period go off then come back on. Can be very annoying if important news is showing.Never noticed this on any of my other Humax boxes that I have had over time.
Have you tried setting the output picture resolution to a fixed value? If you think you have done this check the setting as sometimes it resets itself to Auto.
When in Auto the output is changed to match the input from the source and can cause the picture to go off an on again.
The niggle I had with screen going off for a second when changing apps or channels I seemed to have sorted by turning of AVI infoframe in video and audio as it tells you to do.So all good!