Uwe Brauer wrote:

> The problem is after every restart, my DNS server is deleted from the
> configuration so I have to re configure via tdenetworkmanager the DNS
> server and restart the net, very inconvient.

I am a bit confused now. On both places (/etc/network/interfaces) and in
network manager you can specify the DNS.

See this - looks like similar:
http://askubuntu.com/questions/137037/networkmanager-not-populating-resolv-conf

I think you should keep information only at one place. I now moved
completely to NM since perhaps 1-2 months for similar reasons and do not
experience any issues. However I am using dhcp and never tried fixed IPs.

About the resolv.conf I think it also depends on how you configure the
interface in /etc/network/interfaces. Do you have something like 
        auto eth0
or similar.
I recall vague that there was something ifup/down vs. ifconfig up/down
related.
So perhaps your problem is because of configuration mixing up both.

regards

PS: 
In my case I have a custom script I hooked to /etc/network/if-up.d/
including restarting firewall after changing connection etc. This was
necessary, after moving to systemd. These directories and their files are
honored by NM. It is worth looking into it.
Everything works great! Just keep it simple and clean!