could it be because you have you NIC configured to get IP from DHCP server ?
if so ... dhclient will automaticly update resolv.conf no matter what you've set manually.
by the way /etc/resolv.conf is a symlink for /etc/resolvconf/run/resolv.conf if i'm not mistaken
so i guess you can solve your problem by removing the symlink and create a normal conf file or add a write protect bit to resolv.conf after you've modified it
chattr +i /etc/resolv.conf