Avast Arbitrarily Blocks Good Sites

I find as I am using various sites, such as Facebook and pages linked from Facebook, everything is fine one moment, then suddenly everything stops connecting to the internet. The moment I turn Avast off for 10 mins, everything connects again. Is there a reason why this keeps happening?

What OS/SP ?
What exact version of avast ?
Any other security (related) software installed ? (or was there)