Try turning off the 2.4 radio and see what happens. If you aren't getting good signal strength within 10-15 feet something's wrong. You've already tried changing 5 gig channel so maybe try some further apart. There isn't a whole lot that interferes with 5 gig, some baby monitors/video repeaters but it's rare. I assume you don't have other devices that use "wireless", even if it's not "802.11 WiFi"?