Setting an exact 4:3 resolution (e.g. 1600×1200) should resolve the FOV flicker normally. Also try to run the game windowed if you are running it fullscreen now or vice versa. Some occasional weapon flicker might remain unfortunately.
Also make sure that nothing is running in the background that is taxing your CPU. DirectVR requires an entire core on its own for this game. On mid-range quad core systems without hyper threading (i5), or even worse laptops, that might get difficult if something is running in the background that also uses many CPU cycles.