Most likely it's a setting that can be changed in the configuration for the game. There are quite a few techniques programmers use to generate shadows in games, and yes, some of them can flicker. I would not downgrade graphics drivers as a solution, as you generally end up with lower performance and less resolved issues with games. You probably noticed that not all games exhibit flickering shadows. If you aren't bothered by it, nothing needs to be done. This is not a definitive sign that your graphics card is defective.