[igalan]

Under what circumstances does this happen? If it happens anytime and you have a CRT monitor (Cathode Ray Tube, not LCD-TFT) it may be damaged. If you have a LCD monitor and it happens always when for instance playing a game, maybe you aren't using the accepted frequencies. I assume that you don't have a spare monitor to test, but maybe you can output to TV (through video-out) to check if it's really the monitor or something else.