Does the error appear only when you play games? if yes, then either your in game resolution is set higher than you monitor can handle. Otherwise if it might be a result of your video card going bad. This is error when the monitor either gets no output from video card or a signal it can't handle. Some bad drivers can cause the graphics card to alter the horizontal sync randomly giving this error. Try different drivers. Again overclocking the card sometimes leads to this error, try stock values or even see if underclocking works.