Skyrim is generally very stable for me. Runs smoothly and I hardly ever experience any real problems with it.

That is until I get attacked by ancient dragons. As soon as these guys hit me with their ice-breath the game freezes with a grey screen, with the sound still playing. This is the only time this ever happens, and it only happens with dragon ice attacks. Not other types of dragon attacks. The system won't respond at all and I have to power it off before restarting.

I've only had this problem since Dawnguard. Never happened before that. Anybody got any ideas what may be causing it?