You can also check windows update for the driver
I have a viewsonic vx1962wm monitor and the Windows 7 driver can be found under optional hardware updates, so they might have one for yours also.
Mine works on the standard plug and play driver the same - maybe your using riva tuner or some other third party ATI tweaker application thats causing the problem, if so remove - you can overclock in the control panel. May even need to reinstall the drivers
This is if the drivers dont make any difference - just thought i should mention this.