The xf86 config program is nice but not perfect. If you edit the actual file in /etc/X11 you may be able to see the problem. Normally it is nicely commented (never used mandrake) and you should be able to find the section with the resolution/refresh rate/and screen settings.
The default settings for your monitor *should* work ok, but it's best to find out what they are for sure (vertical sync, horizontal sync, refresh rate and optimal/highest resolution etc)...
It's also possible for some reason it is loading/not loading your drivers right. I guess posting your config file is the easiest way for others to see.