If you are using Nvidia and you have a program like AH that you use the nvidia control panel to manage vsync with and you also delete your video drivers and then reinstall them you sometimes will get an issue where the control panel has a bad link to your program and even though AH might be listed in the window the changes you make will not always apply properly. What you can try is to remove the link through Nvidia Control Panel (reboot) and then Add AH again and set it back up with Vsync on.
