A lot of game developers don't bother to test their games on ATI cards as rigorously as Nvidia cards, so often times ATI needs to hotfix/patch compatibility issues. Sometimes they just screw up as well. Nvidia does the same thing, albeit on a slower pace and with beta drivers. It's just a reality of the business. If you want absolutely stable drivers that rarely have issues then you look at business graphics cards where the drivers can literally spend six months in QA being hammered on.