After hours of investigation during whole week-end we finally found the culprit : one website running Prestashop in production with a huge frequentation had enabled option "PS_SMARTY_FORCE_COMPILE" (since thurdsay 16PM). This option forces smarty to recompile templates on each request and made a huge consumption of CPU that lead to these repeated crashes.
We disabled this option this morning and since then server stopped crashing.
Now we would like to set a cpulimit to avoid this bug to come again. I've found this article : http://www.howtoforge.com/how-to-lim...-debian-ubuntu
but I don't really see how to enable it on my configuration any hint please ?