Monitor not found message is displayed on DG43GT booting time.
I use latest bios with Q6600 and Radeon HD5750 and I know this message appeared only specific VGA card(like HD57xx).
I think this problem will be fixed next bios, but I want to know next bios release date and whether this message can be disabled.
Judging by the change logs, it seems Intel has tried to fix this problem several times. I have no idea when Intel will release another BIOS for your board, or if they are even aware that your problem still exists. Does this message halt the boot process? If not, you can safely ignore it. If so, then look for a "halt on ..." setting that will let you disable "video errors" as a reason for stopping. I don't know the exact wording, but that is a place to start.