The problem is a bit difficult to describe, but I'll give it a shot. It affects all channels. Menus display totally correctly, e.g. the main Roku home menu, the menus that come up when first booting up a channel, even the HUD that comes up when pausing a Netflix video. However, any video that plays is "squashed" vertically, with black bars at the top and bottom. This is video that's supposed to be 16:9 on a 16:9 TV. To put it another way, it looks like the 16:9 video has been letterboxed for 4:3, then that 4:3 picture has been stretched horizontally to fill the 16:9 screen. And again the really weird thing is that if there's a menu also on the screen, even for example the main Netflix screen where a trailer starts playing under the menu, the menu is totally fine, it's just the video itself that's screwed up. It's like it's a bug in the Roku video playing software.
Restarting the Roku 2 fixes the problem, temporarily, but inevitably it comes back. Unfortunately I have not been able to figure out reproduction steps to get it into the broken state, but it always happens pretty soon. It never changes from correct to broken while we're actually watching.
We used to have a similar problem where the black bars would show up on the left and right, "squashing" the picture horizontally, but that would affect the whole system, even the menus. That hasn't happened in a while.
This started fairly recently... maybe with the 8.1.0 update? Maybe not.
Here's the tech info:
Roku 4210X (Roku 2)
Software: 8.1.0 build 4145-04
The Roku is connected to an Onkyo TX-NR646 receiver (to a non-HDCP 2.2 port; when I first hooked this Roku up to this receiver it didn't seem to work at all with the 2.2 inputs), which is then connected to a Sony KDL-65W950B TV.