DNS stops working after few seconds

When I reboot my Ubuntu 14.04 server (with Zimbra mail system) DNS lookup works a few seconds and then it stops.

First I can ping my website but after a minute I cannot, see this:

hans@ubumail:~$ ping www.gso.nl
PING www.gso.nl (217.115.205.81) 56(84) bytes of data.<br>
64 bytes from 217.115.205.81: icmp_seq=1 ttl=122 time=3.35 ms
64 bytes from 217.115.205.81: icmp_seq=2 ttl=122 time=3.19 ms
64 bytes from 217.115.205.81: icmp_seq=3 ttl=122 time=17.3 ms
--- www.gso.nl ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2001ms
rtt min/avg/max/mdev = 3.193/7.961/17.342/6.634 ms

hans@ubumail:~$ ping www.gso.nl
PING www.gso.nl (217.115.205.81) 56(84) bytes of data
64 bytes from 217.115.205.81: icmp_seq=1 ttl=122 time=4.07 ms
64 bytes from 217.115.205.81: icmp_seq=2 ttl=122 time=4.20 ms
64 bytes from 217.115.205.81: icmp_seq=3 ttl=122 time=2.91 ms
64 bytes from 217.115.205.81: icmp_seq=4 ttl=122 time=3.95 ms
64 bytes from 217.115.205.81: icmp_seq=5 ttl=122 time=3.24 ms
64 bytes from 217.115.205.81: icmp_seq=6 ttl=122 time=7.11 ms
--- www.gso.nl ping statistics ---
6 packets transmitted, 6 received, 0% packet loss, time 5004ms
rtt min/avg/max/mdev = 2.912/4.249/7.110/1.362 ms

hans@ubumail:~$ ping www.nn.nl
ping: unknown host www.nn.nl

hans@ubumail:~$ ping www.gso.nl
ping: unknown host www.gso.nl

hans@ubumail:~$ ping www.gso.nl
ping: unknown host www.gso.nl

It looks something is overriding the name server settings after the reboot.

My network settings:

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
     address 192.168.168.20
     gateway 192.168.168.168
     netmask 255.255.255.0
     network 192.168.168.0
     broadcast 192.168.168.255
     dns-nameservers 8.8.8.8

I tried different name servers, that does not help. How to solve this issue?