For anyone experiencing stuttering / buffering on the Amazon Fire TV/Stick I've been recently experiencing issues with larger files and come up with a fool proof solution - with my hardware!
For me I've changed a couple of things.
My orignal Plex Transcoder settings were:
1) Transcoder Quality: Prefer higher speed transcoding
2) Transcoder default throttle buffer: 60 seconds (default)
I've actually found that these settings work better!
1) Transcoder Quality: Make my CPU hurt
2) Transcoder default throttle buffer: 600 seconds (10 minutes)
Additionally, I've then used a freeware app called "Process Tamer" to force the PLEX Transcoder service to always start with "high priority".
Surprisingly I've found that ONLY changing the process priority wasn't enough to completely remove the occasional buffer.
In my mind (looking at CPU utilisation) changing the transcoder to the "Make My CPU Hurt" option seems to maximise CPU usage rather than try to coast along at 20%.
It also looks like that by using increased "quality" in transcoder preferences it's taking LESS time to transcode, as it's not having to compress it at the same time.
This combined with the increased buffer length and using Process Tamer to make sure every time to transcoder process starts it's set to high priority (which it should be in my mind!) seems to iron out all my troubles.
Hopefully this will help someone.
Roy