If your Roku has a "Settings > System > Power > Auto power savings > After 20 minutes of no interaction" setting, try disabling it.
I have had this feature disabled for a long time on most of my Roku TVs & I'm still experiencing the same bug as this person describes. Sometimes I can watch a show or movie just fine, others times it's unbearable, and it occurs on all 4 of the TVs in the house, which makes me suspect that it is a big with the app itself.