Hi till thank you for fast answer.
Unfortunately I already checked all.
Munin is installed, but when apache hangs the log told me nothing. All request are standard and (very strange thing) the traffic is very low (pratically null)!
...i'll take a look to mod_evasive to be 100 sure it cannot be a DOS attack
Mysql is optmized and it does not load too much the system.
just calculated, i have 3 slow query on 9241827 requests (mainly during the night backup)
I supposed the problem could be connected to ispconfig because i checked all before without any clue!
Thank you again for you support. i'll continue to investigate