Certain methods of VSync will require that the frame render exactly when it is supposed to, so if it goes longer you run into an issue where the computer waits for the next frame at that rate. If you look carefully the frame rates you are getting stuck with look something like this: 60fps, 30fps, 20fps, 15fps, 12fps, and 10fps Notice that how all of the rame rates are ratios of your maximum frame rate. 1/1, 1/2, 1/3, 1/4, 1/5, 1/6. This really sounds like frame rate. Here is the explination on stackexchange. Last time I noticed this on my own rig, CCC was forcing VSync on.