Lumberjack Input Ceasing to Function

Hello all!

I'm running several logstash-forwarder instances on remote servers that connect back to a central ELK machine. Events from all logstash-forwarder instances process fine for the majority of the time but seemingly at random logstash stops processing events and generates the following error:

{:timestamp=>"2015-08-17T04:02:43.983000-0600", :message=>"CircuitBreaker::rescuing exceptions", :name=>"Lumberjack input", :exception=>LogStash::SizedQueueTimeout::TimeoutError, :level=>:warn}
{:timestamp=>"2015-08-17T04:02:43.984000-0600", :message=>"CircuitBreaker::Open", :name=>"Lumberjack input", :level=>:warn}
{:timestamp=>"2015-08-17T04:02:43.986000-0600", :message=>"Exception in lumberjack input thread", :exception=>#<LogStash::CircuitBreaker::OpenBreaker: for Lumberjack input>, :level=>:error}
{:timestamp=>"2015-08-17T04:03:21.181000-0600", :message=>"CircuitBreaker::rescuing exceptions", :name=>"Lumberjack input", :exception=>LogStash::SizedQueueTimeout::TimeoutError, :level=>:warn}
{:timestamp=>"2015-08-17T04:03:21.184000-0600", :message=>"CircuitBreaker::Open", :name=>"Lumberjack input", :level=>:warn}

My logstash config is as follows:

input {
lumberjack {
port => 5043
ssl_certificate => "/etc/pki/tls/certs/logstash-forwarder.crt"
ssl_key => "/etc/pki/tls/private/logstash-forwarder.key"
}
}

filter {
if "Connection from UDP:" in [message] {
drop {}
}
if "INPUT pkt died:" in [message] {
drop {}
}
if [file] == "/var/log/messages" {
grok {
match => [ "message", "%{SYSLOGTIMESTAMP:event_timestamp} %{WORD:hostname} %{GREEDYDATA:message_remainder}" ]
}
}
}

output {
redis {
data_type => "list"
key => "logstash"
}
stdout {}
if [file] == "/var/log/messages" {
file {
path => "/var/log/logstash/messages"
message_format => "%{event_timestamp} %{host} %{message_remainder}"
}
}
}

Any ideas as to why this happens? No other errors appear in logstash logs or in logstash-forwarder logs.

What versions are you on?

Logstash - 1.5.2
Logstash-Forwarder - 0.3.1

CentOS 6 on all involved machines

(Working along with davisv) We are also running elastic 1.7.1. Upgrading to logstash 1.5.3 did not help this issue.

Seeing the same issue on 1.5.4 with lumberjack input plugin version 1.0.4.