Emails not processing

This problem has been hitting my email server pretty frequently lately.
It will be working fine and than all of a sudden it appears to stop processing email and the Active queue just builds up.
It keeps building up until I stop zimbra using zmcontrol and than start it back up.
Than it begins to process email again.

When it happens I stop seeing messages regarding status=sent

I see the messages in the log regarding mails being assigned to the active queue, but that's a normal activity.

CPU Usage drops to about 2-3%

I've looked around at a few log files (mainly /var/log/zimbra.log and /opt/zimbra/log/clamd.log and /opt/zimbra/log/freshclamd.log)
I am running Zimbra 4.0.5 open source edition.

Any help would be appreciated.

Thank you.

I'd suggest you upgrade to a more recent version of Zimbra as the one you're running is no longer supported.