use the linux disc 1 to boot into rescue mode, find your /etc/X11/XFree86Config file and under the section "Monitor" change the HorizSync and VertRefresh to whatever it says on the back of your monitor or in the manual.
In other words:-
1. Make a note of what your monitors proper HorizontalSync and Vertical Refresh rate ranges are.
2. Put in disc 1.
4. At the first menu type "linux rescue", mount your linux installation under "/mnt/sysimage" (it should give you a menu that let's you choose).
5. At the console type "cp /etc/X11/XF86Config /etc/X11/XF86Config.bak" (ALWAYS backup just in case).
6. Type "emacs /etc/X11/XF86Config", scroll down to the monitor section with the arrow keys.
7. Change the HorizSync and VertRefresh to whatever it says on the back of your monitor or in the manual.
8. Post how you got on.
Oh, and if something goes wrong and smoke starts pouring out of the back of your monitor
(a joke, this will not happen), you can always get back to square 1 by following steps 2-4 and typing "cp /etc/X11/XF86Config.bak /etc/X11/XF86Config"