[Usafle]

@usafle: True Fullscreen is enabled by toggling OFF the 'Use Fullscreen Windows' -setting in System -> Display -> General.

Ok, checked that, it's OFF, it was on before. No noticeable difference either way however.

Unhook the AVR. Set 3840x2160 in GPU and Kodi. Always, always, always, you have to ONE TIME -----------> *****DURING***** playback, press \.
Now, go to Nvidia control panel resolution. Reset resolution to 3840x2160p @ RGB FULL (should be able to select 12bit) or it will be blank. Leave it blank. It's defaulting to 10 or 8.
Your panel info button should be reporting 3840x2160p @ whatever Hz your source is with match refresh rate ON in Kodi. We are hoping DURING playback your GPU resolution changed to 1920 and correcting it by changing it to 3840.

Below is a screenshot with "change refresh rate ON":



Here is a screenshot with that setting set to OFF:


These were both taken during playback. With "change on start/stop" enabled, it was like I was watching a slow motion video.

Now with the AVR plugged in, Kodi always ... always defaults to 1080p. Even when watching a 4K video, I am STUCK at 1080p with the AVR in the loop. I have tested this by launching the video file and hitting "\" to check on the Nvidia GPU resolution. 1080p. If I manually set kodi to 4k resolution, hit the "\" while file is playing, Nvidia GPU will show 4K. When I hit \ to get back to kodi, display then switches back to 1080p. So we now know it's not "upscaling" to the correct resolution like it's supposed to do?!?!?

That "Jellyfish" video file plays just like the other 4K video files that won't work correctly. AVR in or out of the chain. Resolution set to 4K or not.

You could also try to change the Power Plan in Windows and test drive the High Performance mode. The main trigger here is the Processor Power Management as in HP-mode by default processor is running in full speed and interestingly can remove stuttering even in HW-decoding mode.

Might not the be final solution but worth to check as this has cured some Windows-setups.

Power plan was on High Performance already. Checked just to make sure.