Hi sraper
Go to Settings > Audio & Video > Match frame rate
Change it to the opposite of what it is currently set as, and see it the issue remains or goes away
Match frame rate** is designed to allow the STB to match the frame rate of the content being played out.
Normally keeping the 'Match frame rate' option set to 'Off' which is the default means the box accepts the frame rate difference and is generally not noticeable by most people on most content.
If the 'Match frame rate' is set to 'On' then a blackout can happen when the STB/HDMI reconfigures
**The STB/HDMI is defaulted to 25 frames per second as most content in the UK plays out at 25 fps, however some content, mainly on Netflix and sometimes Amazon Prime plays out of 24 fps (and potentially other rates). This setting allows the STB/HDMI to reconfigure to match the frame rate of the content e.g. 24 fps. Unfortunately changing this config to ON can result in a blackout of around 5-6 seconds if/when the frame rate changes.
(The Match frame rate setting may have been changed if you reset the box, or by a software update)