Router cannot share internet connection

Linksys BEFSR41v4 is the router which is sharing internet between 2 computers. It works until I try to use both computers simutaniously.Say im downloading on the 1 computer, then the other computers internet will be serverly slow and just bogged out.But as soon as i stop using both at a time, the internet will share. Any ideas as to whats happening?

I am having similar problem accept my router won't send a connection to my one Computer!!! I got a new modem and I can't get a connecton from it if I connect it to my NETGEAR router then to my computer. I only get a connection when I connect my Modem directly to my computer. Also, When i plug my modem through my router to the computer my computer picks up a internet connection at 100 Mbps but I just can't get to the Damn internet!! Please If you could be so kind I would really appreciate the help. Can't go onto Xbox Live!! It's torture!! Please help!! THANXXXX