I was having this problem at my old address and I moved (still with Optonline) but have a new modem and I am still having an issue where when trying to get to an embedded link that does not start with "www" it times out during the DNS lookup of the domain. Resetting the Netgear router sometimes works, sometimes it wants the cable modem rebooted.
I use the Firefox browser but I don't think it's the browser b/c this only happens on that home cable connection. It happens on my iMac and I have had it happen on Windows peecees as well. The router gets its connection via DHCP and the internal networked machines use static IP's and look to the router as their DNS server.
I have had this router for sometime and this only started happening a few months back so to me this points to the provider. The router firmware is up to date. It is a Netgear MR814. I have sent emails to the ISP and they have not responded.
I figured I'd post this and see if any other Optonline people have this problem. It's intermittent and usually only manifests when clicking on a link that does not have a "www" in front, just domainname.com/path/to_html_page