Same problem here. After some minutes, usually half an hour or more, the internet is disconnected and Windows fixes the problem saying the default gateway was not found. Router is FRITZ!Box 7490 with Fritz!OS 06.05 Firmware, same with a Telekom Router W920V. Same problem with FRITZ!WLAN Repeater 300E, running FRITZ!OS 06.04. I also tried different channels: Channel 1, 6, 11, 13 - does not help.
It is really annoying. I wish I could change my laptop's network card, but it's not so easy. Please fix that, Intel!
If you need any more information, I'll help where I can.
I have the Intel Wireless-N 7260 (no AC), running on Windows 7 64bit with Intel drivers from 19.07.2013, Version 16.1.1.3
Hardware-IDs in Windows says this here in the top line:
PCI\VEN_8086&DEV_08B2&SUBSYS_42628086&REV_73