Ubuntu server 15.04 losing static IP configuration and gets DHCP address instead

Question

I have a new installed Ubuntu server 15.04.

I configured it with a static IP settings as:

The loopback network interface
auto lo
iface lo inet loopback

The primary network interface
auto eth0
iface eth0 inet static
  address 192.168.1.8
  netmask 255.255.255.0
  network 192.168.1.0
  broadcast 192.168.1.255
  gateway 192.168.1.254
  dns-nameservers 192.168.1.10 8.8.8.8

This works fine after I have restarted the networking service. Or after a reboot. But after some hours, it somehow gets a DHCP address instead. But the above file does not change. So if I restart the service again it will get the static config for a while and then the same thing happens again.

I have also tried to comment out this line in the /etc/dhcp/dhclient.conf

alias {
  interface "eth0";
  fixed-address 192.168.1.8;
  option subnet-mask 255.255.255.0;
}

But no change after this either...

Im really confused here..