Text-mode problem

[Piero Caracciolo <piero.caracciolo@ens.fr>]

When I work in text mode,  "less" and "man" (so again "less", I think) 
don't work properly. The upper part of the monitor (let say 4 to 5 
lines) stays black and the text scrolls only through the rest of the 
screen; when I want to scroll back, only the buffer notices change, not 
the visible text. Thanks for helping.
Piero.