Hi there,
I think http://mirrorlist.centos.org/?release=6.5&arch=x86_64&repo=os should point to: http://mirror.centos.org/centos/6/os/x86_64/ instead of: http://mirror.centos.org/centos/6.5/os/x86_64/ which returns 404. I noticed the same problem for updates, extras, centosplus, contrib also. Unfortunatelly it breaks down updates on some of our servers based on custom image, that has 6.5 in /etc/yum/vars/releasever.