I have the same problem with my hd7970. started in july. i have no idea what's causing it. shadows flicker like crazy. also track side objects seem to glare and flicker a lot, very disturbing when driving.
I have a Crossfire Radeon 5770 and I get grainy alpha textures all over the place, it's horrible.
Using the highest resolution with antialiasing helps a lot though.
Bottom line, this was my LAST ATI/AMD card, and so should be yours.
Next will be Nvidia.