We've had this problem for a while, and as Crayuft said, we've looked into it and it's something of a mystery.
I have this problem about once a week from my work computer and the work-around I've found to work, at least for me, is to use
OpenDNS instead of or in addition to your ISP's DNS server.
One question that has come up is how widespread this problem is, and where it occurs. If you've had this problem recently, I'd be interested in when you were locked out and where you are located, and who your ISP is. You can send an e-mail or private message, whichever is most convenient.
Thanks
Don