Some things to try:
Turn off ALL QoS or Disable Traffic Shaping (DIR only) GameFuel (DGL only and if ON.) options. Advanced/QoS or Gamefuel.
Turn off Advanced DNS Services if you have this option under Setup/Internet/Manual.
Turn on DNS Relay under Setup/Networking.
Setup DHCP reserved IP addresses for all devices ON the router. Setup/Networking
Ensure devices are set to auto obtain an IP address.
Set Firewall settings to Endpoint Independent for TCP and UDP under Advanced/Firewall.
Enable uPnP and Multi-cast Streaming under Advanced/Networking.
What wireless modes are you using? Under Setup/Wireless/Manual.
Try single mode G or mixed G and N?
Preferred security is WPA-Personal. WPA2/Auto TPIK and AES. Some WiFi adapters don't support AES, so you might want to try TPIK only.
Any cordless house phones?
Any other WiFi routers in the area? Use
InSSIDer to find out.
Turn off Short GI and Extra Wireless Protection if you have it. Under Advanced/Advanced Wireless.
Turn off all anti virus and firewall programs on PC while testing. 3rd party firewalls are not generally needed when using routers as they are effective on blocking malicious inbound traffic.
Turn off all devices accept for one wired PC while testing.