Possible that other WiFi and devices in the area could be an issue. Yes, Turn everything OFF. Start with one or minimal devices. I would test with Single mode G with out any security to test. Graduate devices and then turn on security.
These units should work well at G mode at least. Possible that you might need to turn down the routers WiFi if your relaying it with another unit. Since the 628 doesn't need to go that far and the relay unit is doing more of the transmission. Might help.