I think the exact same thing is happening to me...
It happens after the unit has been idle for maybe 30-60 minutes. A reboot always temporarily fixes it. I've already tried a different cable (including the one in the box from Roku), a different HDMI port, and even a different Ultra unit itself (I had bought several, some for other rooms). I've also already tried playing with the screen saver and display resolution settings. No dice.
The issue is only happening on my 2012 Samsung LED TV, but it's a very nice 65" and it would be very expensive to replace. I checked Samsung's web site, and there are no firmware updates available for that TV. I doubt it is HDCP 2.0 compatible, but I have other (smaller) 2012 Samsung TV's in the house that are not having this problem. It should be usable by falling back to HDCP 1.4, based on previous experience.
At this point, I'm probably going to buy a Roku 3 from eBay and just use it, since I wasn't needing the 4K anyway. I'm just disappointed that the newest model is so buggy and unreliable.