im having trouble with a benq 24inch monitor

[imported_K3N]

i recently bought this monitor, http://www.newegg.com/Product/...x?Item=N82E16824014158 , and it has the ability to preserve the aspect ratio when i play xbox 360 on it. I was playing gta4 for the xbox360 at 1280x720 resolution for 3 hours and the monitor was able to preserve the aspect ratio just fine (by putting black bars). Then when i decided to switch the monitor from vga mode to DVI and back to vga mode, the aspect ratio feature stopped working. when i play the xbox360 at 1280x768 resolution and then "switch the monitor from vga mode to DVI and back to vga mode" , the monitor is still able to preserve the aspect ratio. Before i got this 24" monitor, i was playing the xbox360 at 1280x768 resolution on another monitor that didnt have the ability to preserve aspect ratio. I also tried auto configuration on the new benq monitor and it was useless.