Plex crashes when streaming 4k movies

Setup
Plex Media Server - Intel® Core™ i7-6950X CPU @ 3.00GHz - Linux 4.4.30-unRAID x86_64 128GB DDR4 90 TB
Clients Amazon Gen 2 (4k) Fire TV

Issue - I can stream anything thru this configuration accept 4k. Within the first few minutes of streaming a 4K movie Plex locks up my Media Server. The only thing I can do to recover is hard shutdown and reboot.

Troubleshooting - I have tried different 4k movies from different Fire TV’s. I can also stream the same 4k thru my FireTV’s from the same server using VLC. Some buffering and noise but it does not crash.

Any ideas what to try next?

RS