We are discussing this in the 8.1.2 firmware thread. Many of us are having issues with older Wifi devices. Rolling back the firmware does not fix the issue. Something changed which cannot be rolled back. I noticed the comment about N only support. I believe N is supposed to be backwards compatible with G, but I am not sure.
Have you submitted a ticket yet? The more people that report the more awareness we can get.
EDIT: here is the thread: Firmware 8.1.2 Now in GA