I am running the latest firmware (2.05 )on an X9S and there are microstutters every few minutes watching UHD ISO or MKV, bit-perfect rips. Video settings auto, 23.976 is enabled. Even though the Zidoo is reporting. 23.98 output. It “feels” like a classic 24 vs 23.98 frame stutter issue where a frame gets dropped/repeated (I forget which) every few minutes. Is this on the fix list?
Have a look at the X9S 2.05 firmware thread, there is a lot of discussion about this. But yes there is stutter with 4K output.