Thanks a lot for your quick advices.
With nmon, I have the result hereafter :
Code Resource Stats Now Warn Danger
OK -> CPU %busy 2.6% >80% >90%
OK -> Paging size %free 24.0% <20% <10%
Warning -> Paging Space RAM
g 31.7% <50% <10%
DANGER -> Page Faults faults 1045.3 >16/s >160/s
Obviously, I guess this server have a "small" problem of tuning.