if you arent using the latest drivers(both amd and nvidia) that is easily the reason why you may have problems. both companies issued beta driver updates specifically for Far Cry 3
Yup, the drivers fixed everything. Ultra at 4x AA runs fairly smooth. Very playable.
However, I had to physically go search for the drivers, because the driver updater didn't get them. I'm new to the whole PC thing, and I'm used to my consoles updating automatically when they get patches and whatnot.