On 17 April 2012, everyone using our Treadnet TEW-432BRP router noticed that pages would often fail to load, displaying "Server not found", or partially load (always broken). We have to refresh the page repeatedly to get it to load fully. Uploading anything, including posting a comment, often froze and required multiple page refreshes. Since it occurred on every website we visited, the fault is in the ISP or our router. Connecting to the router using Ethernet didn't fix the problem.
I was also able to play TeamFortress2 for hours when connected to the wireless router, without any lags or internet problems.
Connecting directly to the modem fixed the problem.
We contacted Mediacom ISP and they claimed it was our router's fault, so my dad replaced it with another TEW-432BRP (we used to have three of them, two of them in the unopened retail boxes). Then the modem went down for the next few hours, requiring a system reboot from the ISP.
Today, we finally got the network back up, but the "Server not found" or partially loaded pages still persist.
EDIT: We use a variety of web browsers, ranging from IE8, Chrome, Firefox and Aurora (alpha version of FF). Often times Pingtest will inform me that its unable to connect to a server, and when it can, it gives an A rating.
EDIT2: Youtube's video loadings work fine, except for the page loads.
I'm assuming your router auto detects the DNS settings from your ISP? Perhaps their DNS servers are having issues? Try manually setting your computer(s) DNS servers to 188.8.131.52, and 184.108.40.206 respectively. These are public DNS servers run by Google. They are are very reliable for uptime, and should prove whether your ISP's DNS servers are the issue.
All of this is also assuming of course that your ping tests were done while attempting to ping a website address, rather than an actual IP address.