Plex buffering

I've had this issue several times recently, and I've yet to figure out what is causing it.  I'll be watching a video on Plex and somewhere in the middle or after the halfway point it will start buffering.  The Plex Media Player log has lines like this:

2017-07-27 22:06:03 [ INFO  ] JS: %c[Commands] Executing persistPlayQueue 
2017-07-27 22:06:04 [ DEBUG ] PlayerComponent.cpp @ 526 - cache: Cache is not responding - slow/stuck network connection? 
2017-07-27 22:06:04 [ INFO  ] JS: %c[Commands] Executing persistPlayQueue 
2017-07-27 22:06:05 [ INFO  ] JS: %c[Commands] Executing persistPlayQueue 
2017-07-27 22:06:06 [ DEBUG ] PlayerComponent.cpp @ 526 - cache: Cache is not responding - slow/stuck network connection? 
2017-07-27 22:06:06 [ INFO  ] JS: %c[Commands] Executing persistPlayQueue 
2017-07-27 22:06:07 [ INFO  ] JS: %c[Commands] Executing persistPlayQueue 
2017-07-27 22:06:08 [ INFO  ] JS: %c[Commands] Executing persistPlayQueue 
2017-07-27 22:06:09 [ INFO  ] JS: %c[Commands] Executing persistPlayQueue 
2017-07-27 22:06:09 [ INFO  ] JS: %c[Timeline] playing, 1669751/1978977 
2017-07-27 22:06:10 [ INFO  ] JS: %c[Commands] Executing persistPlayQueue 
2017-07-27 22:06:11 [ INFO  ] JS: %c[Commands] Executing persistPlayQueue 
2017-07-27 22:06:12 [ INFO  ] JS: %c[Commands] Executing persistPlayQueue 
2017-07-27 22:06:13 [ INFO  ] JS: %c[Commands] Executing persistPlayQueue 
2017-07-27 22:06:13 [ DEBUG ] PlayerComponent.cpp @ 526 - cplayer: Enter buffering. 
2017-07-27 22:06:13 [ INFO  ] PlayerComponent.cpp @ 399 - Entering state: buffering 
2017-07-27 22:06:13 [ INFO  ] JS: %c[Player] 0% buffered 
2017-07-27 22:06:13 [ INFO  ] JS: %c[Commands] Executing persistPlayQueue 
2017-07-27 22:06:13 [ INFO  ] JS: %c[Timeline] buffering, 1673588/1978977 
2017-07-27 22:06:13 [ INFO  ] JS: %c[PlayerController] State change buffering 
2017-07-27 22:06:13 [ DEBUG ] PowerComponent.cpp @ 53 - Enabling OS screensaver 
2017-07-27 22:06:13 [ INFO  ] JS: %c[Player] 4% buffered 
2017-07-27 22:06:14 [ INFO  ] JS: %c[Commands] Executing persistPlayQueue 
2017-07-27 22:06:15 [ INFO  ] JS: %c[Player] 8% buffered 
2017-07-27 22:06:15 [ INFO  ] JS: %c[Player] 19% buffered 
2017-07-27 22:06:16 [ DEBUG ] PlayerComponent.cpp @ 526 - cache: Cache is not responding - slow/stuck network connection? 
2017-07-27 22:06:19 [ INFO  ] JS: %c[Timeline] buffering, 1673588/1978977 
2017-07-27 22:06:23 [ DEBUG ] PlayerComponent.cpp @ 526 - cplayer: End buffering (waited 10.122276 secs). 
2017-07-27 22:06:23 [ INFO  ] PlayerComponent.cpp @ 395 - Entering state: playing 
2017-07-27 22:06:23 [ INFO  ] JS: %c[Commands] Executing persistPlayQueue 
2017-07-27 22:06:23 [ INFO  ] JS: %c[Timeline] playing, 1673588/1978977 
2017-07-27 22:06:23 [ INFO  ] JS: %c[PlayerController] State change playing 
2017-07-27 22:06:23 [ DEBUG ] PowerComponent.cpp @ 58 - Disabling OS screensaver 
2017-07-27 22:06:23 [ INFO  ] JS: %c[Commands] Executing persistPlayQueue 
2017-07-27 22:06:24 [ DEBUG ] PlayerComponent.cpp @ 526 - cache: Cache is not responding - slow/stuck network connection? 

While in the Plex Media Server log I'll see messages like this around the same time:

Jul 27, 2017 22:06:02.733 [0x2b40d6600700] VERBOSE - We didn't receive any data from 192.168.1.62:39513 in time, dropping connection.
Jul 27, 2017 22:06:06.010 [0x2b40d6600700] VERBOSE - We didn't receive any data from 192.168.1.62:39515 in time, dropping connection.

My configuration is:

Pentium G4620 (Kaby Lake) @ 3.70GHz

16GB DDR4 2400 MHz

Plex Media Server docker (linuxserver.io) latest Plex Pass version

Plex Media Player on a Mac mini, latest Plex Pass version

Hard-wired gigabit ethernet from server -> EdgeRouterX -> client

There are several other docker containers running (Plex, PlexPy, Radarr, Sonarr, Jackett, Deluge, Transmission, Observium, cAdvisor) but I've never seen any of them chew up significant resources for more than a few seconds.

I'm direct streaming the video.  The most recent one to get stuck buffering had a bitrate of 3653 kbps, but I've had the same issue with higher or lower bitrate videos.  I've also direct streamed 12 Mbps vidos without issue.

When the buffering starts, I'll quickly try to SSH in, run diagnostics and see what's up, but I'm not seeing any obvious clues.  The web UI will often lag for a minute or so around these times.  

I can perform an iperf test between the client and server and see ~940 MBit/s consistently.

I can run 'htop' and 'iftop' and 'iotop' on the server and nothing seems to be chewing up resources.

Then things will mysteriously go back to normal after a few minutes and playback will be fine.

tower-diagnostics-20170727-2208.zip