Dealing with resource issues can be difficult. As such, you will want to gather more information and investigate what might be causing the resource issues. Some commands that might help you are:

sudo iotop -abkotn 3 
sudo top -bn1 -o %CPU|head -n 25
sudo netstat -atn|grep :80|awk '{print $6}'|sort|uniq -c

If the iotop command is not installed, you can install it by running this command:

sudo apt-get install iotop

If you do not already have it on your droplet, you might also wish to install the sysstat package. This package includes sar, which logs resource usage for the droplet in timed intervals. It is quite useful for seeing what resources were being used when a droplet crashes or has issues. You can install it via:

sudo apt-get install sysstat

Being it is running via Passenger, you might also want to check the Apache and Passenger logs to see if any recurring errors are causing issues, or if any bad requests are occurring.

Hope it helps,
Jason Colyer
DigitalOcean Platform Support Lead