I am pretty sure that this problem is unique to the battlefields only. I cannot remember running into it in the main game and I have been through the game 2 1/2 times so far. I have run into it in the battlefields on several occassions though.
IMHO, the problem may be related to the problem where items lose their properties. That bug seems to screw up the monsters in the dungeons, causing them to either fight barehanded, or (again my opinion) not attack at all, which seems to put the game in slow motion.
Whatever the reason, the Larian folks are aware of it, and it has been posted several times on the forum.
I have been playing in the "after-game" dungeons since the new patch came out and after 8 straight dungeons have not seen the problem occur again (maybe just lucky though).
