@djwave:
If I'm not mistaken you can switch between 15KHz and 31.5KHz on the fly in the PPC bootmenu by just pressing space on the keyboard. In 31.5 KHz mode, the 1084 monitor will be unable to display the image, so you will have to plug in your VGA monitor and then press on the "Save" button to make 31.5KHz mode the default.
/Patrik