Router/Playstation3 Issues?

Ok I have a Cisco Linksys Router WRT120N. I set it up and my PS3 receives internet, the problem is the lag/connection issues I have when I play Battlefield Bad Company 2, I know it's not the game cause im the only one with problems. I also narrowed down that when my computer is turned off, I don't experience lag. Could the problems be with my Internet Service provider or with the computer itself, How can I increase the connection, or change the bandwith? Any help with be greatly appreciated