Hi Pablo,
could you give also settings of the MySQL engine ? usually the default tuning of MySQL is quite low for high load handling. Maybe the crash comes from blocking calls to the database API that comes unanswered due to logical limitations in settings...
We ran a sucessfull test for 1750 simultaneous users on a 3 x 1 x QuadCore culstered plant and we had to retune Mysql to accept 5000 input connections (in our case, Moodle virtualisation solution we made require an extra connection per page).
It may only be an hypothesis....
Hope it can help.
cheers.