I have same exact problem on Comcast (used to be adelphia) with motorola surfboard.
I had this problem in the beggining, I could only access admin on router if I disconnect from cable modem. If i tried to access admin setup with cable modem hooked up, the dlink would just reboot everytime I tried to login in.
Now it has changed.... now I try to connect t dlink, i get the login page... I enter llogin and submit.... the router just reloads the login page, no error or wrong password message or anything, just reloads the login page... with cable hooked up and cable not hooked up... after hard reset.. every scenerio... just reloads login page.. FRUSTRATING!!!