This may help some having the issue with no video, I found this fix on another thread & it worked for me:
"Re: Audio but black screen for video.
To me, it sounds like you're set to an incompatible resolution.. go into the Roku settings and change the resolution to 720p and see if that resolves it. It may be trying to run at 1080p and your TV isn't capable."
Thanks for the suggestion, but it doesn't fit my symptoms. If video resolution was incorrect, ALL TV viewing through the Roku would work incorrectly or not work at all. Whereas my and others' symptoms are that the Roku works fine, then suddenly the black-screen with OK-audio problems occurs suddenly, and goes away only after total power-cycling of the Roku box.
I think that, like PCs running Windows, the Roku box simply suffers from bit-rot that accumulates over time (months to years), and that a hard reboot (power-cycling) cleans up, so the cycle can start again.
I am having a similar problem. My screen goes black every 15 seconds or so when watching Acorn TV or Netflix but not when watching regular cable tv. I have unplugged. Rebooted. Refreshed the network connection. Reinstalled Netflix and acorn on my TCL roku tv. So the problem has to be roku related and not an hdmi cable.