This thread caught my eye because I also discovered that if I turn off Web Shield and Secure DNS it would fix a problem I was having.
After researching some non-Windows tasks, I found AMDquickstream, which does end-to-end traffic shaping of IP packets. After uninstalling the package (Control panel → Programs → Uninstall a program) and reboot, and then reinstalling Avast (I had temporarily switched to another antivirus package), the problem seems to have gone away.
I have no idea if this is what you are experiencing, but the symptoms look similar.
Good luck!