I have a Roku Ultra with an Enhanced Remote (TV volume and power control) connected to a Samsung TV. The TV started powering down and back up spontaneously a few weeks ago. At first it happened intermittently. But over time it started happening more regularly until it was happening every few minutes. At first I thought it happened only while watching Roku video, but then I discovered that it happened to all TV input sources.
I checked the Roku remote's batteries and they were nearly dead. After I replaced the batteries the problem went away!