Check your driver settings...try setting your AA and AF to application controlled. Seen reports in the past that running over 4x has shown strange behavior on the newer cards or drivers. Might want to try lowering the multiplier in game too.

There is also a texture memory issue with the newer generations as well...forget exactly which families and driver version level it started showing up--the workaround was to use the LoD streaming options, may get more bang if you use lower details on distant objects as well. The pop-in effect can be annoying, but if it stabilizes the game until they get the kinks worked out of the drivers/firmwares....