This is usually as sign that it is near time for a new monitor. I am not sure if it is new or not. If it is new, you may consider returning the monitor and telling them the problem that you are having.

In addition, it is entirely possible that your video card is also causing the problem. It is less likey than your monitor but still a possiblity. You may try the higher refresh rate and then push back your graphics acceleration/color bit size/resolution.