Question: I have a Macbook Air 13" (mid 2012) on which I installed Windows 10 via Bootcamp a while ago. It all runs smoothly and works fine except for the WiFi. 

The internet connection breaks frequently (which means that I am not able to access the internet). I am though still connected to the WLAN network at any time (often it says "no internet, secured"). Disconnecting and reconnecting to the WLAN network is the only way to get it back to working. I noticed the same behaviour in different networks. Other devices do not show interrupts.

When using OS X the connection is fine all the time.

I already reinstalled the broadcom driver and deactivated the option that the system can disable the device for energy management reasons (I don't know the exact name of this option in english, sorry).

What else can I do? Did anyone have the same issue or a similiar one? Thank you in advance for responding.

Answer: 
Deinstalling Cisco VPN Client fixed my problem. 

There seems to be a problem with updating from Windows 8 to Windows 10 when Cisco VPN Client is installed (<https://superuser.com/questions/947651/no-network-connection-after-upgrading-to-windows-10>). 

Although my Windows 10 was a fresh install, deinstalling Cisco VPN Client helped me getting a stable internet connection.
While I do not have specific experience with Win 10 on this model of MacBook, some people have good success with the OEM manufacturers drivers instead of the ones Apple provides.

So you would go into device manager and look for the make and model of WiFi hardware. (For example Broadcom 3502) and then go to the manufacturers website and download the latest Win 10 drivers for that make and model.

The tricky part may come if device manager only shows hardware IDs. In that case you may have some success Googling those particular numbers.

Again I don't know if that will solve your specific problem but I have solved similar problems with other versions of Windows on other Macs using this technique.