You mean like a bolt hits your screen? Set the correct frequency for your monitor then, or get an uprated HDMI 1.3 cable if you use HDMI.
My monitor frequency at the monitor menu says 67.5 hz but I setted the mnini to 60hz, as 67.5 is not an option. I use DVI cable, and a hmi>DVI adapter. Could this frequency problem be causing it ?