I tried with Slax and Gnoppix and the problem was that my monitor wasn't set correctly if the OS uses X.org.
If Linux uses XFree it can correctly setup it.
So I have done a bit of "cut, paste and renaming" from the XFree config file of Gnoppix to my Slackware xorg config file
Now, I can use the correct resolution/Hz
thanks a lot for yours help !!!!!!!!!!!
and special thanks to mods for the double post problem (sorry, excuse me again)
Happy New Year to all Linux Question users