Deployment PhusionPassenger CPU 100%

Hi, I deployed my Meteor app on Ubuntu 14.04 machine using Phusion Passenger to provide load balancing.
My problem is that from time to time one of processes takes 100% usage of processor and never goes down. I must restart the process.
What is strange, this situation appears more ofter during the life of the app. I mean: 3 month ago it didn’t happen. 2 months ago it was happening every couple days. Now it happens twice a day.
If I change number of processes in passenger config everything works good again - but there is the same scenario. During the life of the app this strange situation happens more often.
The app doesn’t grow too fast. There is still the same amount of users and database doesn’t grow too much. Any ideas what causes the problem?