I was looking thru the settings menu when I was changing the screen def from auto to 1080p for my TVs. Looking in the Advanced System Settings I found a choice for Advanced Display Settings. In there is a choice for Auto-adjust display rate. It was turned off in all 3 E4K units. So, I turned it on. It said it makes the refresh rate the same as the native refresh rate. I'm not sure what that really means. But suspect it means is matches the broadcast refresh rate. If that is true, then I could see where the screen could freeze because it is refreshing at a different rate than the broadcast for a moment and then stays out of sync.
I realize that I changed two things, the resolution setting to 1080p and turned on the auto-refresh rate. So, if the problem does not go away, I will not know which one solved it. But that can be figured out by turning on of the changes back to where it was and seeing if the problem returns.
I'll report back if I am successful. If I am, maybe Roku will offer me a job. I know the work load sure won't be excessive. 😜