I have this issue on both my first gen ATV 4k and my 2021 model, on an LG B7 and an LG C1.
Basically, all my 1080p videos play choppy. They usually start out smooth, and then Lauren choppy after 10 minutes or so of playback. My 4k videos play fine. Other apps, like TV+, Hulu, HBO all play their content perfectly smoothly. Plex exhibits similar behavior.
A common fix I see suggested is to turn on match frame rate. This does fix the problem, but it introduces other issues. More specifically, match frame rate causes lip sync issues in some apps, but not all. As a result, it cannot be corrected with the lip sync tuning on my AVR. And even if it could, having to change the audio delay every time I switch between my Apple TV and PlayStation would get real old real quick.
I think what is happening here is that infuse and Plex are doing something during playback that messes with my TV’s ability to detect 24 fps content in a 60 hz container. I know this TV (LG C1) is fully capable of smoothly displaying 24 fps content from a 60hz signal, as evidenced by the fact that Infuse does work properly for 10 minutes or so after a fresh boot.
This issue has been driving me crazy for a while now. It was not a problem until I think around the time Infuse 7 came out.