Some lost frames with 6K VR (180 deg.) videos because of wrong codecs settings?

Hey.
I have Valve Index and I experience some performance issues when playing 6k video / 180 degrees with SteamVR. I am currently using the SKYBOX VR player because it seems to work best anyway. I have K Lite Codec Pack Standard in the latest version. Selected codec: NVIDIA CUVID.

Problem: Every 20-60 seconds it loses a few frames. It is not a big loss, but very annoying. It happens randomly, so it's not a problem with the quality of the vid. itself. The problem is less visible with 5K (HEVC) and doesn't exist with 4K. I have quite decent PC: RTX 2080 SUPER, Ryzen 3700X, fast 32 GB RAM and SSD on PCI-E. Win 10/64 bit - freshly installed system. Software and drivers: all updated. It seems impossible to me that it's the hardware fault. When it comes to codecs, I have probably reviewed all the threads and tried all the suggested solutions that are on steamcommunity. Among other things, I also installed https://www.microsoft.com/en-us/p/hevc-video-extensions-from-device-manufacturer/9n4wgh0z6vhq - nothing changed.

I have no idea what else I'm doing wrong. In general, how even can you debug such a thing?
Has anyone encountered a similar problem?

Regards, C.

You should try "DXVA2 native" as the hardware decoder, or try without hardware decoding. This of course only has effect if that player uses DirectShow codecs.