Try
http://www.pcpitstop.com as well. They test your entire machine including your internet connection. If they find a problem they give you links to their Autofix solution. My cable connection was a little slow and their Autofix increased my receive buffer size to one more suitable for a cable modem.
http://www.pcpitstop.com/pcpitstop/autofix.asp
They also give you the option to return to the default settings if you wish.
Good luck!