Nodes dropping out of cluster due to 'null node' exception

Having a problem with ElasticSearch 0.90.10 where nodes will drop out of
our cluster with the following Exception:

org.elasticsearch.ElasticSearchIllegalStateException: can't send request to
a null node
at
org.elasticsearch.transport.TransportService.sendRequest(TransportService.java:179)
at
org.elasticsearch.transport.TransportService.sendRequest(TransportService.java:173)
at
org.elasticsearch.action.support.single.shard.TransportShardSingleOperationAction$AsyncSingleAction.perform(TransportShardSingleOperationAction.java:179)
at
org.elasticsearch.action.support.single.shard.TransportShardSingleOperationAction$AsyncSingleAction.start(TransportShardSingleOperationAction.java:125)
at
org.elasticsearch.action.support.single.shard.TransportShardSingleOperationAction.doExecute(TransportShardSingleOperationAction.java:74)
at
org.elasticsearch.action.support.single.shard.TransportShardSingleOperationAction.doExecute(TransportShardSingleOperationAction.java:49)
at
org.elasticsearch.action.support.TransportAction.execute(TransportAction.java:63)
at
org.elasticsearch.client.node.NodeClient.execute(NodeClient.java:92)
at
org.elasticsearch.client.support.AbstractClient.get(AbstractClient.java:179)
at
org.elasticsearch.action.get.GetRequestBuilder.doExecute(GetRequestBuilder.java:112)
at
org.elasticsearch.action.ActionRequestBuilder.execute(ActionRequestBuilder.java:85)
at
org.elasticsearch.action.ActionRequestBuilder.execute(ActionRequestBuilder.java:59)
at
org.elasticsearch.action.ActionRequestBuilder.get(ActionRequestBuilder.java:67)
at
org.elasticsearch.river.routing.RiversRouter.updateRiverClusterState(RiversRouter.java:132)
at
org.elasticsearch.river.routing.RiversRouter$1.execute(RiversRouter.java:107)
at
org.elasticsearch.river.cluster.RiverClusterService$1.run(RiverClusterService.java:110)
at
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)

We then also see the following status

curl http://localhost:9200/_status
{"error":"NullPointerException[null]","status":500}

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