That NVR bandwidth is limited to 80Mbps, so depending on the resolution, # of cameras, FPS, and bitrate, you could be overloading the NVR.
It could also be a codec issue.
If you are running a codec (like H265+ or H265 with AI codec, etc.) then change it to plain codec.
If that isn't it, try 10FPS, H264, 4192 and see if the problem goes away. If it does, then you know it is a bandwidth issue.