Check your hosts file on you computer. Make sure no entries have been made. Scan for spyware/malware.
As a test, remove the router from your network and see if you still have the problem.
If it goes away, then I would do a factory reset on the router. You'll have to reconfigure it if you had any personal entries. If the problem still persists, try a different router.
If you still have the problem connected directly to the modem and your computer is clean from spyware it could be a problem outside your network which there isn't much you can do.