Postgres Replication Issue

We have master server on one Datacenter, and the Replicate is in another
Datacenter

Intermittently, on the replicate server, we see this as below.

“2018-12-19 07:24:25 UTC: : @: [25775]: [2-1] FATAL: could not receive data
from WAL stream: server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.”

We have all DB (master and replicate settings) in place for streaming
replication, and we are using the same settings on 100+ replicates and there
is no issue except this.

I would appreciate if someone would help here.

1) we have wal_files on the master
2) hot_standy is on

When we initially start the replicate server, it streams, and if in case
streaming is lagging, we have restore_command to restore from master
pg_xlog_archive.

so, most of the time, we don't have issues, but sometimes we have this error

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