Hello.
At the end I solved the connecivities issues following the instructions in this link: http://logz.io/blog/elasticsearch-cluster-disconnects/
To make a short recap: there is a bug in the ubuntu network driver that causes some random packets loss. In the syslog of the server I got messages like this:
xen_netfront: xennet: skb rides the rocket: 19 slots
The procedure to solve the problem is to disable the the Scatter / Gather ability in the network interface.
//To see if scatter-gather is ON on eth0:
ethtool -k eth0 |grep sca
//to set it to off:
ethtool -K eth0 sg off
@Christian_Dahlqvist: I will follow the recommendation regarding the allocated java heap, thank you for noting it.