Can't connect Vista Media Center to my xbox360 using wlan

I am trying to connect my Sony Vaio with Vista and Media Center to my xbox360
using WLAN. My Xbox is connected to my WLAN router using cable and my Vaio
is connected using WLAN. The installation of the xbox360 as Media Extender
worked well but after 1 minute of being connected the WLAN connection is
terminated and the WLAN network adapter on my Vaio is deactivated. If I
activate the network adapter everything works again but then the problem
repeats itself.

According to the network diagnostics in Media Center I have enough speed for
ordinary TV but not for HDTV. I have tried to switch of energy saving of the
network adapter and also the automatic identification of faster WLAN
connections.

If I connect my Vaio to the WLAN router using cable everything works
perfect. Anyone has a clue to what may be the problem