Hmm, I'd say it is either a setting in your router (but having reset it, pretty much rules that out) possibly a cable problem, or something phsically wrong with the router.
Do you have 2 Computers? If so you could try pinging one another. other then that I'm fairly stumped myself.
Can you bypass the router? hook directly into the modem? If so then I dunno I'd call up linsys or the store you bought it from and see if you can take it back and get a replacement. but from the sounds of it, you purchased it a while ago.
Sorry I haven't been much help, troublshooting ftw =/