Unable to send message through cluster sender - Tomcat 6

Question

We have 2 Tomcat 6 servers in a cluster. Occasionally we get this error, say after every 4 days:

INFO   | jvm 1    | main    | 2012/01/15 17:43:26.586 | SEVERE: Unable to send message through cluster sender.
INFO   | jvm 1    | main    | 2012/01/15 17:43:26.586 | org.apache.catalina.tribes.ChannelException: Send failed, and sender is disconnected. Not retrying.; Faulty members:tcp://{192, 168, 100, 127}:4000; 

or this:

INFO   | jvm 1    | main    | 2012/01/15 17:36:44.707 | SEVERE: Unable to send message through cluster sender.
INFO   | jvm 1    | main    | 2012/01/15 17:36:44.707 | org.apache.catalina.tribes.ChannelException: Operation has timed out(60000 ms.).; Faulty members:tcp://{192, 168, 100, 126}:4000; 

This occurs on both nodes. When this occurs, our sites eventually goes down.

The 2 nodes are behind an Apache HTTP web server. It connects with the 2 Tomcat nodes using AJP with Reverse-Proxy.

We've been trying to locate the source of this problem for ages, but we've haven't found anything concrete. The cluster configuration in server.xml of the respective Tomcat nodes is the following:

<Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster"
                    channelSendOptions="6">

    <Manager className="org.apache.catalina.ha.session.DeltaManager"
                       expireSessionsOnShutdown="false"
                       notifyListenersOnReplication="true"/>

    <Channel className="org.apache.catalina.tribes.group.GroupChannel">
        <Membership className="org.apache.catalina.tribes.membership.McastService"
                          address="228.0.0.4"
                          port="45564"
                          frequency="500"
                          dropTime="30000"/>

            <Receiver className="org.apache.catalina.tribes.transport.nio.NioReceiver"
                          address="@machine.ip.address@"
                          port="4000"
                          autoBind="100"
                          selectorTimeout="5000"
                          maxThreads="6"/>

            <Sender className="org.apache.catalina.tribes.transport.ReplicationTransmitter">
                  <Transport className="org.apache.catalina.tribes.transport.nio.PooledParallelSender" timeout="60000" keepAliveCount="0"/>
           </Sender>

        <Interceptor className="org.apache.catalina.tribes.group.interceptors.TcpFailureDetector"/>
        <Interceptor className="org.apache.catalina.tribes.group.interceptors.MessageDispatch15Interceptor"/>
    </Channel>

    <Valve className="org.apache.catalina.ha.tcp.ReplicationValve" filter=""/>
    <Valve className="org.apache.catalina.ha.session.JvmRouteBinderValve"/>

    <ClusterListener className="org.apache.catalina.ha.session.JvmRouteSessionIDBinderListener"/>
    <ClusterListener className="org.apache.catalina.ha.session.ClusterSessionListener"/>

Could anyone help us out plz?

Thanks!