ES disconected while running on EC2

Hi,

I setup 3 ES instances on EC2 using 3 small instances: instance1,
instance2, instance3
Loaded 70 docs into it.
I notice that while i was running search test or indexing data 1 or 2
instances were disconnected from cluster
I don't know why it disconnected? please advice me what to do to prevent
this problem.

Here are log when 1 instance disconnect.
in INSTANCE3:
2013-05-19 23:48:15,363][DEBUG][action.admin.indices.stats] [instance3]
[nndex1][1], node[rc3l-dm0Ra-UTYyXM4DG8A], [P], s[STARTED]: Failed to
execute
[org.elasticsearch.action.admin.indices.stats.IndicesStatsRequest@5ab8c80a]
[2013-05-19 23:48:15,411][INFO ][cluster.service ] [instance2]
removed
{[instance2][rc3l-dm0Ra-UTYyXM4DG8A][inet[/IP2:9300]]{master=true},},
reason:
zen-disco-node_failed([instance2][rc3l-dm0Ra-UTYyXM4DG8A][inet[/IP2:9300]]{master=true}),
reason transport disconnected (with verified connect)

in INSTANCE2:
[2013-05-19 23:48:01,698][INFO ][cluster.service ] [instance2]
master {new
[instance1][px2Qw0UVTLSZvj-YVNFZaQ][inet[/IP1:9300]]{master=true}, previous
[instace3][mnJ2oA02R0-Hzsr_RmxWpQ][inet[/ip3:9300]]{master=true}}, removed
{[instance3][mnJ2oA02R0-Hzsr_RmxWpQ][inet[/IP3:9300]]{master=true},},
reason: zen-disco-receive(from master
[[instance1][px2Qw0UVTLSZvj-YVNFZaQ][inet[/IP1:9300]]{master=true}])

in INSTANCE1:
[2013-05-19 23:48:01,558][INFO ][discovery.zen ] [instance1]
master_left
[[instance3][mnJ2oA02R0-Hzsr_RmxWpQ][inet[/IP3:9300]]{master=true}], reason
[do not exists on master, act as master failure]
[2013-05-19 23:48:01,565][INFO ][cluster.service ] [instance1]
master {new
[instance1][px2Qw0UVTLSZvj-YVNFZaQ][inet[/IP1:9300]]{master=true}, previous
[instance3][mnJ2oA02R0-Hzsr_RmxWpQ][inet[/IP3:9300]]{master=true}}, removed
{[instance3][mnJ2oA02R0-Hzsr_RmxWpQ][inet[/IP#:9300]]{master=true},},
reason: zen-disco-master_failed
([instance3][mnJ2oA02R0-Hzsr_RmxWpQ][inet[/IP3:9300]]{master=true})

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