Hi,
I'd like to add that I'm having the exactly the same issues as GarthJ described, down to connecting at a good rate for an indeterminate amount then an eventual slow-down to between 6-12Mbps which is virtually unusable. If I reset the network adaptor it will then go back up to a healthy 150-300Mbps for a while, before repeating. Sometimes I'll use Windows 7's "Diagnose" network tool which will often identify a different issue as being responsible and rarely giving a consistent reason for the reduction in speed (Problem is modem, DNS, access point?...), but "Diagnose" always ends in auto-restarting the network adaptor and is satisfied that it has fixed the issue. Until of course it slows down minutes or hours later. I'm not seeing any correlation between uptime or data downloaded during 'connection period' so I'm stumped. I have the Intel Dual Band Wireless-AC 7260 device which is attached to an mITX motherboard (Gigabyte GA-F2A88XN-WIFI).
Is there any resolution on this issue? I have the latest drivers, and really need this fixed, as I'm not forking out for a PCIE Wireless adaptor while I wait to use Intel's onboard solution (which is why I purchased the motherboard).