Airport Extreme 2.4 Ghz suddenly very slow
I have been running the latest Airport Extreme for several months now. Very reliable and fast, with speeds around 35 to 45 Mbps. Both in the 2.4 and 5 Ghz channels.
This morning I had to switch off the AE as an engineer from the cable company had to check the cable for an unrelated issue with the TV. After I turned on modem and Airport, the wireless connection was very slow (below 2 Mbps). My first action was to reset the modem and the AE, check the wires etc but to no avail. I then made a wired connection from the AE to the iMAc and this connection was perfect. Hence the internet connection itself appears to be fine.
The I switched to the 5Ghz network and speed was as usual - fast. At 2.4 still slow. Returning the AE to factory settings did not help as well.
Can someone please advise what to do next. The physical location of the device did not change, I did not make changes to the settings. And what a coincidince would it be that a neighbor decided today is the day to install a wireless router so strong that it blasts away the AE signal (the AE is 4 meters away).
iMac (27-inch Mid 2011), OS X Mountain Lion (10.8.2)