BUG #14714: long running sessions from remote instance seems to hang some times

The following bug has been logged on the website:

Bug reference: 14714
Logged by: Josef Machytka
Email address: josef(dot)machytka(at)gmail(dot)com
PostgreSQL version: 9.6.3
Operating system: Debian jessie
Description:

On pg 9.6.* we have sometimes strange problem with some sessions. We
experienced it from bash scripts, golang program and node.js applications.
Environment are Google Compute Engine instances with Debian 8.

Sometimes some session goes into some kind of "drowsy" state and runs
incredibly slowly. It causes some very low disk IO and runs like 20x slower
then usual. This happens mainly when database is under heavy load and it
happens randomly to different tasks. It happens randomly to both connections
from local instance (cronjob running on instance with PG) and remote
connections from other instances.

When we restart task it runs normally. I cannot see any changes in process
priority in OS, process switches between sleeping and running state and I do
not see any locks or waits in pg_stat_activity or pg_locks. Usually all
other processes finish OK and this "drowsy" one runs "for ever..."

We implemented timeouts for PG tasks into our programs to restart tasks but
it very annoying and we have to use long enough timeouts anyway so it delays
tasks.... It happens from time to time also to dblink connections inside pg
function so we had to rewrite our programs which were running without any
problems on PG 9.5. Problems started when we switched to PG 9.6 and we
experienced them on all minor versions - we currently run 9.6.3 and problem
is still present...

Do you please have any advice what to check or what setting might help?