I am having the same problem as the author. Tried reinstalling the Arma 2 games and Dayz but the problem persists. I can only choose low or normal texture packs too for some reason. Fairly sure the Very High was available before I swapped to Windows 8. If anyone has a tip on how to fix the terrain flashing/become transparent I would be grateful. Meanwhile, I am going to experiment what I can do to fix this on my own.