Sounds more like a VRAM thing than regular system RAM. If you post a dxdiag file we can take a closer look. You should be able to directly upload the .txt file here. If you haven't used that command before it's super easy to search and run from start menu then hit "save all information" to make a text file you can post here.
My guess is there's an issue there and could be in relation to any settings you have in game. If we're running the game absolutely full bore crysis grade visuals but dont have enough vram to DO that well there's the issue.