It happens on any channel, and began happening maybe a month ago or so. There are many other users with the same problem - I have checked via Google and have seen others posting here too. I have not bought a new HDMI cable, but did reverse the cable, and no change. I have an LG 55" 4K model TV that is less than 4 years old.
I have tried everything except resetting my Roku. I didn't set it up originally because I am the opposite of techy, but I think either that and/or a new HDMI cable is the next try. Not sure though why an HDMI cable would go back in a year or less of use!