Are you running a standalone Vid card or just using the onboard? You also didn't mention what manufacturer the monitor is and whether it has auto settings options. Many, such as myself, use standalone vid card(s) depending on original system capabilities and to acheive better video functionality overall.
My LCD montior changes the resolution every hour for 60 seconds. I have it set at the recommended resolution and every hour it automatically decreases the resolution and enlarges the screen for 60 seconds and then returns to the correct setting. I am running Vista . We have replaced the video driver and it still does it. All the setting seem correct. Any clues how to get it to stop?