Some of people or device may use traffic a lot at channel 7-10.
So you can test it a night(nobody use the Air time).
Or find another AP and test it again
For AIR check( wifi ap channel info) go to google play and download "wifi analyze" then you can see the air.
thanks for this hint! (Apple products + wireshark is an good option, too)
I just find out there may be a HW issue. Done here for now...