Elasticsearch process on ubuntu 16.04 stops - no error log

I have multiple elasticsearch instances on a server.
everyone once in a while (day or two) one of those instances just disconnects from the cluster.
there is nothing in the error log to indicate what is happening.
The java instance is still running happily with active systemctl status.
So i try a restart and it just hangs in a deactivating state.
Only after doing a kill -9 to the process(es) does it finally restart.
I can't find anything in system logs either.
Not sure where else to look?

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