L4d2 connection problems

I can't seem to connect two computers on the same network to a server online at the same time.

Cable internet, to a linksys wrt54g to two computers all wired. I have messed around with some of the routers settings but nothing seems to help, reset to default etc.

I have played l4d1 with the same router/setup with 3 computers before with no problems connecting or playing.

I have tried resetting the modem, router, renewing/releasing in ipconfig, rebooting etc. Same thing still happened, connection failed after 10 attempts for one of us and the other one gets into the server, not always the same computer that gets in.