Most times I've seen this it has been a fault in the connection between PC and monitor. Have you checked that the cable between monitor and PC is definitely connected correctly? Also, check the pins on the plugs at either end of the monitor cable and make sure none have been flattened. If the pins look fine (you'll tell if not, they typically get squashed to the side or bottom of the connection), and the cable connection seems fine, then try borrowing someone else's screen and see if it occurs with theirs. If so, its your graphics card, and you could try opening your PC up again and reseating it (unscrew, pull out, plug back in again) - long shot though.
