Tried to play a game last night, and the problem is MUCH worse. The game freezes at the movie screen; sometimes it will eventually pass, and sometimes not. It has frozen completely in the lobbly, so that I had to CAD out. Twice. During the game, there are regular freezes now, lasting ~10-15 seconds, then a bit of play, then another freeze, in a pretty regular cycle.
I don't have a firewall, other than Windows. I have AVG antivirus running, although this has never been a problem in the past.
I will try to launch a game later this afternoon, and see if anything has changed.