Why does my monitor report an error when I run at a lower resolution?

I’m running a Samsung Syncmaster 2433bw monitor and a Radeon 3850… typically, on the desktop I’m running 1920×1200 at 60Hz resolution.

However on newer games I run at 1280×800 in the game to get smoother gameplay and reduce the load on the video card… when I do so, however, after a couple of minutes the monitor blacks out and reports “Not optimum mode”, “Recommended Mode 1920×1200 at 60Hz”... as if to say it wants me to run at the higher load resolution.

The image will come back and I can clear the error message but then after a couple of minutes… black screen and error message again.

Why won’t it let me run at the lower resolution? Is there a way to force the lower resolution without damaging the monitor or blacking out and reporting the error?

Thanks in advance for any help…