What browser are you using?
Try Opera or FF? If IE 8 or 9, set compatibility mode and test again.
What Hardware version is your router? Look at sticker under router.
What
Firmware version is currently loaded? Found on routers web page under status.
What region are you located?
Are you wired or wireless connected to the router?
Some things to try:
Setup DHCP reserved IP addresses for all devices ON the router. Setup/Networking
Ensure devices are set to auto obtain an IP address.
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.