Sometimes the resolvers in the server cannot find a server. You can update the DNS resolvers (/etc/resolv.conf) to rotate between a number of nameservers to try and resolve the issue. Add google nameservers to the mix with your Host resolvers.

# nano /etc/redolv.conf

nameserver 8.8.8.8
nameserver 69.xx.xx.xx
nameserver 8.8.4.4
nameserver 69.xx.xx.xx