My environments is: I have one desktop using DWA-160 setting on 5GHZ and one laptop using intel wirless setting on 2.4Ghz, and I have a logitech Z5450 2.4GHZ cordless spearker, these things are in one room, and Dir-855 is in another room maybe 15meters away. they are in one floor.
I used to use one linksys G router, and both of my laptop and desktop running extremely slow connections when I turn on the Z5450, and back to normal speed when I turn it off. that's why I recently bought Dir-855. After I set it up and update to 1.12 firmware, my desktop's connection become normal after I setup to 5Ghz, (but have another issues I've posted in another post) but the laptop's connection is just like what you guys said, in the log of the router, it keeps saying 2.4GHz linked, and associates etc. So I don't know if this may help you guys to figure something out. I think it something about the equipment close to your area running the same frenquency.