If you have Comcast or Optimum Online, it might not be a bad idea to check out the interface MTU on your public interface:

% ip link show eth1
4: eth1: <BROADCAST,MULTICAST,UP> mtu 576 qdisc pfifo_fast qlen 1000
    link/ether 00:48:54:8e:e0:30 brd ff:ff:ff:ff:ff:ff

See that 576?  I'm not sure why this happens, but I'm speculating it's due to a misconfigured DHCP server on the ISP's side of the house.  RFC 879 specifies 576 as the IP maximum datagram size.  However, we know that the link type is Ethernet, so it should be 1500.  Also, if you're using dhclient, it's probably actively requesting the option "interface-mtu" along with a couple others.  You can remove this option from dhclient.conf, and bounce the interface.  Here's a portion of Debian"s dhclient.conf:

#prepend domain-name-servers 127.0.0.1;
request subnet-mask, broadcast-address, time-offset, routers,
     domain-name, domain-name-servers, host-name,
     netbios-name-servers, netbios-scope, interface-mtu;
#require subnet-mask, domain-name-servers;

The "netbios-name-servers" and "netbios-scope" options can, and probably should be removed, too.  I can't imagine why anyone would need that data from a residential ISP.

Just as a test, I configured dhclient to request the "interface-mtu" option on an Ubuntu box here.  My DHCP server (ISC dhcpd, 3.x) refused to return anything for the option, so I'm guessing the cable companies are using something other than ISC for their DHCP server.

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