This problem is not solved. Thread should remain open.
Under Ubuntu 12.04 and 14.04 LTS, same phenomenon and I'm using the latest version of i-mscp at the moment (1.1.11).
When it crashed, error 500 is displayed on all domains and sub-domains hosted on the server. The fix is to restart php5-fpm.
I notice that sometimes it hangs during the night but I encounter the problem also during the day so I don't think is cron related.
The only servers that suffer of this bug are the one that has large traffic websites, with more than 2500 unique visitors / day.
@Nuxwin, give me please a set of instruction to look for when the php5-fpm is stuck. I'll post them here...
Thanks!