This problem only happens to me when I’m using the ISP with EnterNet 300 and it’s definitely a “mtu” related problem and more or less due to the tuning of ISP itself. You won’t notice it until you are downloading email or web page or anything which never get through.
If you happen to have problem getting through a EnterNet 300 broadband connection, you may try this “sudo ifconfig ppp0 mtu 1400” in Terminal where ppp0 is the connection to your broadband modem.
The last number can be anything lower than the default 1500. You can check the available connection by issue “ifconfig -a” in Terminal.