It sounds like you have been extremely thorough, you probably already thought of this, but just in case – does the monitor have another connector you can use besides VGA? DVI or HDMI? According to the M58 specs, the computer has Displayport and VGA. Update: The 9800 GT specs say it has “HDTV” (I assume that’s HDMI and Dual Link DVI connectors. With a VGA cable, are you using the M58’s motherboard video and bypassing the 9800GT?