I just got a brand new 24" asus monitor, and this problem just started happening. It will be at regular brightness for 1 second, and then it will go dim for 1 second, and it continues to cycle dim/bright. I just got it, so it shouldn't be a problem getting it replaced, but does anyone know why this might be happening?
If it is an LED LCD monitor, look for a setting called "dynamic contrast". If it has this feature, you can turn it off and the monitor should stay at a consistant level of brightness. Provided that it's working correctly. Also, do as MJ5150 said and try loading the driver to see if that helps.