Setup Reserved (Static) IP addresses on the router for your devices.
Ensure DNS IP addresses are being filled in under Setup/Internet/Manual.
Ensure Advanced DNS Service is disabled if you have this option.
Ensure DNS Relay is enabled.
Set Firewall NAT to Endpoint Independent for both TCP and UDP.
What wireless modes are you running? Mixed? Single?
Try single mode G to test, then try mixed G and N.
Set for WPA and TPIK only and test. I might try testing with out any security for a period of time and see if it exhibits the same. Turn it back on.