[CentOS] Apache stops without evident cause

OS = CentOS-6.4 Uptime = 4 days, 1:31, httpd-2.2.15 SELinux=enforcing audit2why -l -a = nil report

Since upgrading a public web service host from dedicated hardware running CentOS-5.x to a KVM virtual host running CentOS-6.3 I have experienced intermittent ( one every couple of months) halts by the Apache web service. There is never any trace left in any of the logs, there are no SELinux avc's recorded, and the rest of the system remains up. It is just the httpd service that stops.

I have an automatic restart job in place so this is not a show stopper. But I am concerned that this behaviour is pointing to a fundamental problem somewhere.

Has anyone else experienced this? Are there any ideas as to what could be causing this?