The intranet killer… Skype.
If you are connected to a home network or even a little office network that share an internet connection that looks unstable. Try first to close Skype and then read the following article about how Skype can use your bandwidth and block your web access.
Entering via UDP, TCP, or even TCP on port 80, Skype is usually very successful at passing typical firewalls.
Powered by ScribeFire.