Before re-installing, I looked  for the config file, but did not found   /etc/x11 directory !?!..... 
Now, looking for it again, I see a X11 (with a capital X, at the beginning of the list ....) and sure enough the config file is there .... <img src="/images/graemlins/confused.gif" alt="" /> ..
Dumber than dumb ! I really need to get used to Linux ..... !!       <img src="/images/graemlins/blush.gif" alt="" /> 
Anyway, I'm not sure I would have been able to modify the correct parameter...
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Alain