I've only seen this after this weeks patch (0.75 I think)

After playing for a long time, without warning, the dialog boxes just display as text on pure black.

The game is still running and responsive, but very sluggish and jumpy.

I can make a save game and exit the normally but it takes a while.

I have seen this quite often since the patch (Which incidentally has stabilised the game considerably for me on 32bit XP).

After this problem happens I always reboot the PC to sort out any garbage collection/OS problems and reload the game and carry on normally.

Thanks for reading.