I can't see how this can happen intermittently, web shield would be scanning all downloaded http port 80 traffic and you shouldn't notice an effect, it should be either slow all the time or normal.
Because you say closing browsers and re-opening sometimes works to clear it if this was truely a web shield bug then that shouldn't have any effect. I suspect some sort of interaction with another application, because by disabling web shield resolves it as you take one side of any potential conflict away.
The web shield doesn't block, it scans and alerts to any infection, but it isn't designed to block.
What other security based software do you have that might have an effect, e.g. anti-spyware applications, SpySweeper, Spyware Doctor, etc. ?
What changed two weeks ago, there has been a program update on 30 April 2007 so at the latest you are likely to have updated this (on the 7 day program update cycle) is on or about 5-7 May, around two weeks ago. Unless you had any other application installations or updates around that time ?
If it is related to the latest update then perhaps you should try a fresh/clean install:
It would probably be best to first download the latest version of avast
http://www.avast.com/eng/programs.html and save it to your HDD, somewhere you can find it again. Use that when you reinstall.
Ensure you are off-line, uninstall, reboot, install latest version you downloaded, reboot.
