What ISP Service do you have? Cable or DSL?
What ISP Modem do you have? Stand Alone or built in router?
What Firmware version is currently loaded?
Ensure DNS IP addresses are being filled in under Setup/Internet/Manual?
Turn off Advanced DNS Services if you have this option.
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.
Are any power management options set on the PCs NIC?
What connection speeds does your PC NIC support?
Also might try setting the LAN connection speeds on the router to manual, try 100 first then 1000.
Ensure Cables are in good condition, use Cat6 if possible. Swap out cables to be sure.