"OUT OF SCAN RANGE" error on bootup. How do I change these settings?

Hey everyone, I'd appreciate some help with this because it has everyone here at our office stumped. We have a Dell OptiPlex GX110 running with one of those big, hulking, 21" black Dell monitors (model #P1130 I believe) that's getting the OUT OF SCAN RANGE error after the Windows 98 load screen every time it boots up, but from then on it's usually fine. Sometimes we have to restart and try again, but most of the time the computer is workable. We just want to fix it because boot up time is nearly doubled becuase of it. Dell's troubleshooting gave me this:

<<OUT OF SCAN RANGE
This indicates that the input signal is not supported by the monitor's specifications.
If you are replacing an old monitor with this monitor, reconnect the old monitor. Then adjust the computer's graphic board so that the horizontal frequency is between 30 - 130 kHz, and the vertical frequency is between 48 - 170 Hz.>>

I know how to change the refresh rate (that's the vertical frequency, isn't it?), and I have it set at 85Hz (we run a 1024x768 resolution). My question is, how do I change the other frequency? I've looked everywhere for this setting! I've downloaded new drivers for video card and monitor, and no setting for it anywhere.  However, even if I did adjust the other setting, would it make any difference, or should I pair the computer with a different monitor?  Can someone please help?  Thanks.