ActiveMQ consumers stop accepting messages

Hi,

I have two ActiveMQ 5.8 instances setup in a network of brokers. Each
instance is hosted on a separate machine running Ubuntu 12.04 with 8GB of
memory. After one week in production, we started seeing consumers that
wouldn't accept messages. In the web console I can see the messages enqueue
and messages in the dispatch queue, but the messages are never consumed in
the server application. Restarting the consumer doesn't fix the problem.
When I tried restarting the ActiveMQ instance it failed to shutdown --
needed to kill -9 the pid.

I believe the problem is on the broker or in the CMS library, but I can't
seem to track it down. Has anyone seen a problem like this before?

===================================================================
Application topology, request/response pattern w/temp queues, not
persistent:

ASP.NET/NMS <-- producer (x350) --> ActiveMQ 5.8 (x2) <-- consumer (x120)
--> C++/CMS
Win2003/IIS6 Ubuntu 12.04/OpenJDK 7
Ubuntu 12.04

===================================================================
Portions of activemq.xml:

<amq:policyEntry queue=">"
optimizedDispatch="true"
lazyDispatch="false"
producerFlowControl="false"
strictOrderDispatch="false">
<amq:deadLetterStrategy>
<amq:individualDeadLetterStrategy queuePrefix="DLQ."
useQueueForQueueMessages="true" />
</amq:deadLetterStrategy>
<amq:dispatchPolicy>
<amq:roundRobinDispatchPolicy />
</amq:dispatchPolicy>
<amq:messageGroupMapFactory>
<amq:simpleMessageGroupMapFactory />
</amq:messageGroupMapFactory>
<amq:pendingQueuePolicy>
<amq:vmQueueCursor />
</amq:pendingQueuePolicy>
</amq:policyEntry>

<amq:memoryUsage>
<amq:memoryUsage limit="64 mb"/>
</amq:memoryUsage>
<amq:storeUsage>
<amq:storeUsage limit="100 gb"/>
</amq:storeUsage>
<amq:tempUsage>
<amq:tempUsage limit="50 gb"/>
</amq:tempUsage>

===================================================================
JVM memory:
ACTIVEMQ_OPTS_MEMORY="-Xms4G -Xmx4G"

Thanks,

Dave