I've now tried Wireless_16.10.0_Ds64.exe (that's the drivers only version for Windows 7 64bit) since March 22nd 2014 and it has worked like a charm since.
I've successfully downloaded Gigabytes, connected to various Routers and AccessPoints and never had a single interruption since. Drivers are in default state (HT Modus not deactivated, just everything default).
Driver date says 28.01.2014
Driver Version says 16.10.0.5
So if anyone still has any disconnection issues ("default gateway is not available"), maybe try out this driver version. Maybe you can still find it on the Intel Website, else I have uploaded it here splitted in 2 files.
I won't update drivers now.. Never change a running system. 
@ Thanks for the tip. In the past, driver updates were not successful, but this was a good one.
edit: typo