Having problem with display

Hi there, I am a G5 2.7 Dual user, I run on a samsung 19" monitor, but recetnly i keep getting a blank page on the screen with a small box saying not in optimum mode, recommended mode 1280 x 1024 60hz. I have never had such problem before and the problem only surface once in a while. Can somebody help to rectify this problem. Thank you very much!

Open System Preferences, click on "Displays", then select the recommended resolution/refresh rate.

I know that, but the problem only surface when i boot it up or half way thru using the com. Is there a problem with my os?