Hi Headhunter06,
Thank you for using the Apple Support Communities! I understand you are noticing a flicker during video playback within the Apple TV app on your iMac. I am happy to explore this with you.
You did some great steps to help rule out a few possibilities! Thank you for sharing these in your post. Are you noticing a flicker in your display during playback within any other apps, or just the Apple TV app?
I would like you to force quit the Apple TV app: How to force an app to quit on your Mac - Apple Support, then restart your iMac: How to turn your Mac on or off - Apple Support. If the same flicker remains, continue to the next steps.
Let's have you boot up into safe mode. It’s a way to start up your Mac so it performs certain checks, repairs, emptying of system cache, and prevents some software from automatically loading: Use safe mode to isolate issues with your Mac - Apple Support. Once in safe mode, check to see if the same behavior continues when playing videos with the Apple TV app. If your Mac runs normally in safe mode, restart and log in as normal to see if that helps resolve the issue.
If the issue remains, I will have you test a new user account next, to isolate if this is a user-specific issue, or a system-wide issue. Here’s how: How to test an issue in another user account on your Mac - Apple Support. Based on the results in the new user, you could continue to the section titled 'If the issue doesn't happen in the other account' or 'If the issue happens in the other account' to continue troubleshooting.
Keep me posted on the results.
Regards.
