WLAN truncate after DVI display plugged in

May be someone knows this problem.

MacBook Pro Retina with Mountain Lion

After I plug in the DVI Monitor over Thunderbold, the WLAN connection breaks.

The WLAN logo is still there, I have still the assigned IP address from the WLAN router.

But any connection to outsite, a ping or a web site call does not work.

After I unplug the connector, some seconds later, the ping is working and Im able to call any web site.

I can repeat this each time.

Any idea ?

greetings

Sven