I wish I had an sgi setup so I could go through the steps exactly, but it has been a couple years so bear with my explanation.
You set the hz/resolution for your video card higher than your monitor actually supports. So you need to boot into single user mode. When you first boot up the sgi it brings up a screen that says hit esc or something like that to configure. Then pick single user mode or shell option(I forget what it is called but it is like the second to the bottom one). Then it will boot up in a shell only. Put in your root password. Then type something like: "setmon -h 1280_1024x70". setmon is the command to set the resolution, then the "1280x1024" is the resolution and the "x70" is the hz. Change those to the settings to one that your monitor supports. I forget the switch, but it is something like setmon -h that tells it to permanently keep the setting you give it. Just type setmon by itself and it will give you all the switches to use.