This issue might be caused by DRAM frequency scaling, which changes RAM frequency at runtime based on usage. Mainline Linux supports DRAM frequency scaling on RK3399 via the `CONFIG_ARM_RK3399_DMC_DEVFREQ` kernel config, but mainline is missing a patch that prevents the frequency from changing while a frame is being scanned out. That patch is needed because changes during scanout can cause glitches like the ones you describe in the RK3399's Video Output Processor (VOP).
To check if that's the issue, you can try forcing the RAM frequency to the maximum by running the following:
echo 'performance' | sudo tee /sys/class/devfreq/memory-controller/governor
That'll result in higher power usage, but it should eliminate the glitches until you next reboot, as the RAM frequency won't change anymore.
