Brand new AppleTV 4K box cuts out

The TV screen going black momentarily indicates that the TV is adapting to changing video modes while bouncing between the Apple TV home screen, an app, a screensaver, or a program you're starting to watch. If this happens in the middle of a program or movie while you're watching it, I would be surprised (and it would indicate a more serious problem). The video mode-switching frequency can be minimized by correctly setting your TV and ATV4K devices to agree on the best available video format(s) that both support and staying there.

You don't identify a specific LG model so I don't know its full capabilities, but I'll guess it's similar to or better than my older/cheaper LG model... In your LG TV's settings, make sure that Connections > Device Connection Settings > SIMPLINK (HDMI-CEC) is turned on and I suggest turning on Auto Device Detection and Auto Power Sync (for full & proper operation of the ATV Remote). Under Advanced, enable the deep color mode for the HDMI port your ATV4K is connected to, and in general enable all the spiffy HDR10/HDR10+/Dolby Vision like capabilities that your TV supports.

Next, go to ATV4K Settings > Video and Audio > and under VIDEO select Reset Video Settings. Then, enable HDR. Then set Chroma to 4:2:2. Note that if your TV has more advanced abilities than mine, you may see additional options regarding Dolby Vision, etc. NOTE (this is important): resist the urge to turn on Match Format. Leave it off! It is the constant matching of subtle differences in content that causes your TV to constantly go black as it adapts to different frequency ranges and frame rates. I prefer to set ATV4K to match the best capabilities of my TV (and stay there). Finally, if you've got a nice iPhone, scroll down to CALIBRATION and do the Color Balance process.

Hope this helps.

I have this problem with a new TCL tv. I tried everything and nothing works, the problem now happens within an hour of starting to watch anything on the Apple TV and then is random, switches off for a couple of seconds, the TV indicates a new signal (with format info and then restarts from where it left of. There was a previous thread that spoke of the same problem, hard to find, Apple TV 4K Keeps Cutting Out - How Do I … - Apple Community, the thread shut down but without any resolution. I also have an LG CX and it’s fine with the ATV. So I shift the Apple TV between the two and only the TLC is problematic. The previous thread spoke about problems with Sony and Panasonic TVs, so this is wide spread. Suspect happens with certain hdmi TV adapters. Happy for any insight, have tried every single ATV configuration option.

I believe you're seeing a mismatch between the ATV4K settings and the TV's settings. your LG likely has features the TCL doesn't so swapping the ATV4K device without reconfiguring both ends of the connection will be problematic.

1. With the ATV4K connected to the TV with a good HDMI cable, use the TV's settings menu to enable all HDR-related high-end features on the HDMI port your ATV4K is connected to.
2. Reset the Video Settings on the ATV4K and then turn on/verify HDR features that your TV supports.

Fingers crossed. HDR10+ and Dolby Vision do make periodic adjustments over the course of a movie depending on changes in content.

If you take the ATV4K out of the picture (no pun intended :-) and watch the content using just the TV's webOS apps, is everything good?

Thanks, have tried all that, all high end features turned on Dolby vision, hdr10+, hdmi advanced turned on, checking Dolby atmos on atv, indicates it’s fine. Cables top end hdmi 2.1. Setting on hdmi advanced on TV as opposed to auto helped, problem stopped happening every 10 minutes, now happens every hour or so. Using the native android app on the TCL incl AppleTV+ app works fine including Dolby vision / atmos viewing - no black outs. The TCL has a hdmi adapter with 2 x hdmi 2.1 and 2 x hdmi 2.0, which I think other manufacturers use, wouldn’t be surprised is Apple TV does not play well with these.