Channel Width set for Auto 20/40Mhz or try 20Mhz only.
Try setting a manual channel to a open or unused channel.
Any other WiFi routers in the area? Link> Use
InSSIDer to find out. How many?
Possible your Phone could me a source of the problem.
I would turn off all WiFi devices and then turn on one and test. Graduate turning on each device.
Maybe someone can review your router settings with you using Link>
teamviewer if your interested. Its safe and secure.