What Hardware version is your router?
What Firmware version is currently loaded?
Ensure DNS IP addresses are being filled in under Setup/Internet/Manual?
Turn off QoS options.
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.
Ensure devices are set to auto obtain an IP address.
Set Firewall settings to Endpoint Independent for TCP and UDP.
Review the Gaming and Gamefuel sticky for help with QoS and XBL. It works great.
Maybe someone can review your router settings with you using
teamviewer.