[CentOS] Need help with an odd issue I am experiencing

I have a server with dual nic's running centos 5.2 1 nic connects to the 
cable modem, which has 15m d and 2m up, but speed tests barely come back 
at 5mb down including road runner's internal speed test. I hook up my 
laptop to the same modem, same port, using the same cable and I get 17mb 
down. I have ruled out iptables, nics (because same result on both 
nics), cables, cable modem(with a brighthouse rep) any suggestions would 
be greatly appreciated. Thanks.