With vegetation at "high", "very high", "ultra" settings vegetation is flickering all the time. When you set it to "normal" it doesn't because it removes vegetation.
The problem been there since the game was released and happen with all Radeon HD6900 series (with all drivers to the date):
- see reports here (everybody with same 69XX hardware): https://support.rockstargames.com/hc/communities/public/questions/203751347-Flickering-vegetation-on-PC
- and here is a video showing the bug: (GTA V grass flickering issue with AMD Radeon HD 69XXX - YouTube )
Please Drivers devs take a look, Radeon 6900 series is not that old, thanks!