I have a dir-615 version b. 2.25 firmware.
I had been using it for a while with no problems with my dsl connection. Recently, we upgraded to comcast cable. When the router is using the cable connection, 90% of web pages don't load. Google and yahoo load fine. This site loads fine actually. But the vast majority of websites don't work- from blogs to major corporations web pages. On a lot of websites, adobe.com for example, the HTML will load fine, but the CSS will not load at all, so all i see is black and blue text and a few images.
I read on another thread of someone having a similar problem who only had to lower the MTU to make it work... I was not as fortunate.