I've seen this before too. I don't know what causes it, but it seems like the ethernet just isn't negotiating properly. To clear it out, I've forced the interface to renegotiate by either disabling and re-enabling the inteface (unless you are behind it - don't lock yourself out
![Smile :-)](./images/smilies/icon_smile.gif)
) or you can also uncheck autonegotiation and click apply, then go back and check it again and hit apply. That seems to force renegotiation without too much of a chance that you'll lock yourself out of the router.