I can't notice a pattern either, as the web shield would either work all of the time or not at all. You could also check out your ISPs support/status pages and see if they are reporting any outages, etc.
What add-ons do you have installed in firefox ?
You could try running firefox (safe mode), it disabled all add-ons and see if that makws any difference.
If none of the above makes any difference, since firefox have now released version 8, it might be an idea to uninstall the current firefox and do a clean install of firefox8 and see if that makes any difference.