I to have the same problem.
did everytheng you did includind a fresh load of my os.
OS is Windows 8.1
NIC is Killer wireless-1202 Network Adapter
AV is Webroot secure anaware v18.104.22.168
the area I am having problems is in Trenton NJ
Near the Main Trenton Police Station.
I Bleave thay are causing interfeaince, I Ma be wrong.
Or it is the 3 new WIFI Tellephont towers that were just put up in the last 6 months.
Can you see the xfinitywifi SSID? Is it somehow disabled in My Account?