Yes I know a little late but just got a 4k TV. Noticing some stutter using Channel app on 4k FireTV stick that did not see with regular HDTV. I guess this means FireTV not able to process mpeg2 TV channel at 4k resolution output? I don't think the FireTV can output 1080i but is it possible to limit the Channels app output to 1080p without making a global change to the FireTV 4k output?
If you change the global resolution does it actually fix the problem you're seeing?
Changed settings in Channels app from software to hardware decoding and much better now.