Turn on karmic dice. It will make the rolls less random (or more random, if you perceive the game to be bugged).
You're not the first person to swear there's something wrong with the dice. Wormerine isn't the first to speculate there isn't. So, please. Send Larian a bug report, maybe there *is* something your game has in common with people's who also claim this problem. It's the best thing you can do to eliminate it.
Turning on karmic dice should also help you hit more (and be hit more by enemies, keep that in mind). It favours glass canon builds, as higher AC (19+) will be hit disproportionately more than they should. That doesn't mean increasing your AC will stop being helpful, it's just less effective than without karmic dice.
