This is for ubuntu server edition 8.1, so i dont have the network manager installed.
I found the solution on another post.
It looks like the problem only is because of my isp provider (cablevision)
so in this file - /etc/dhcpd3/dhclient.conf
i removed interface-mtu so that when i make a dhcp request to my modem it doesn't return the isp mtu setting and it automaticly sets it to 1500,
request subnet-mask, broadcast-address, time-offset, routers,
domain-name, domain-name-servers, domain-search, host-name,
netbios-name-servers, netbios-scope, interface-mtu;
I do appreciate your post because it sent me searching and thinking in the right direction. Big swoosh and woot for you.