Thanks for the suggestions - the video card was new at Christmas (I treated myself!), and the problem existed before the latest patch. I've been playing around a bit with video settings and so far the problem only seems to occur when I run the game in Windowed Mode. If I run it full-screen, the lock-ups seem to go away (so far) - but then I lose sound when I alt-tab out of the game and back into it. I have read other reports of that problem with sound so will investigate, but I'd prefer to run in Windowed mode, obviously without the lock-ups. I can't think of an obvious reason why running it in a window would cause this problem - I do use Compiz though, so maybe I'll try disabling that and see how I get on.
