Help with new router and Xbox lag

Okay so heres is what I have. I just got a new cable modem and router. I have comcast basic high speed. The modem is motorola sb5120 and the router is d-link dir 655. The xbox is connected through a wired connection. I upgraded this from a netgear CG814WG wireless cable modem. I upgraded from this because there was a 5 ip limit. on the netgear I had a dmz and port fowarding set up and it almost never lagged. Now with the new setup i start to lag about 4 minutes in and lag out of every single game. I have the xbox's ip set to the dmz. I have tried port fowarding, but when I do the xbox for some reason doesn't connect to live. i have tried ports 3074 and the port dlink provides in the internet based software. Also the internet itself seems to be slower. Currently only 1 computer and my xbox are connected. I can download a torrent easily without any problem. Could the router be faulty? Any help is greatly appreciated. Thanks