if it is the IVAD cable, then use the old poor-mans trick of wire and aluminum foil wrapping around the IVAD cable, then electrical tape up the wrapping with a wire attached, extruding from the tape to be grounded. the shielding failed in the early runs of the cables. This could be a fix if wiggling the cable doesnt change the results.

Usually, the raster bounces and shifts all over the place if this cable is at fault. Try it. a large majority of 700mhz eMacs were affected with the IVAD cable issue from what I see.

If that dont take care of the issue, then time to troubleshoot the pincushion circuit components for opens and shorts.