Mule FTP polling dies silently

Hi,

I am using Mule 1.4.3 running in Tomcat 5.5 to poll several FTP
servers.  After some hours (12-30) of inactivity, Mule will stop
polling one or more of them, but not always the same ones.  For
example my log (catalina_2008-08-25.log) will have

26248:DEBUG 2008-08-25 15:50:42,485 [org.mule.providers.ftp.FtpConnector.connector.ftp.0.receiver.6]
: >>> retrieving client for ftp://anonymous:mule@ftpServer1/
26249:DEBUG 2008-08-25 15:50:42,485 [org.mule.providers.ftp.FtpConnector.connector.ftp.0.receiver.6]
: === get pool for ftp://anonymous:mule@ftpServer1/
26250:DEBUG 2008-08-25 15:50:42,485 [org.mule.providers.ftp.FtpConnector.connector.ftp.0.receiver.5]
: >>> retrieving client for ftp://anonymous:mule@ftpServer2/
26251:DEBUG 2008-08-25 15:50:42,485 [org.mule.providers.ftp.FtpConnector.connector.ftp.0.receiver.5]
: === get pool for ftp://anonymous:mule@ftpServer2/

and then ftpServer2 will disappear from the logs and files on that
server will not be polled, though the other servers continue to be
polled.  

Remark 1: It may be useful to know that "receiver.5" does not reappear
in catalina_2008-08-25.log, or catalina_2008-08-26.log, either.

Remark 2: I am able to manually put and get files to the unpolled machine.

My config has this:

  <mule-environment-properties>
    <connection-strategy
      className="org.mule.providers.SimpleRetryConnectionStrategy">
      <properties>
        <property name="retryCount" value="-1" /> <!-- Retry forever: -1. -->
        <property name="frequency" value="600000" />
        <property name="doThreading" value="true" />
      </properties>
    </connection-strategy>
  </mule-environment-properties>

and the endpoints look like

      <inbound-router>
        <endpoint address="ftp://anonymous:mule@ftpServer1/">
          <filter pattern="*.zip"
            className="org.mule.providers.file.filters.FilenameWildcardFilter"/>
          <properties>
            <property name="pollingFrequency" value="60000"/>
            <property name="passive" value="false"/>
            <property name="outputPattern" value="${ORIGINALNAME}"/>
          </properties>
        </endpoint>

        <endpoint address="ftp://anonymous:mule@ftpServer2/">
          <filter pattern="*.zip"
            className="org.mule.providers.file.filters.FilenameWildcardFilter"/>
          <properties>
            <property name="pollingFrequency" value="60000"/>
            <property name="passive" value="false"/>
            <property name="outputPattern" value="${ORIGINALNAME}"/>
          </properties>
        </endpoint>
        ...

Any help would be appreciated, thanks.

Glenn