Site not loading via DO - high CPU usage

I’m using Ghost on $5 Digital Ocean droplet - mrhack.io

It has few thousands posts, around 10K monthly visits.

I recently migrated from WordPress. But, sometimes the website is not loading. Seems it is due to high CPU usage.

Some help from DO support:

The most common reason of this issue would be MySQL stopped running. It stopped or failed to start as a result of not enough memory. Restarting the server is only a temporary fix and will not solve the problem.

Any advice? I know maybe I need to upgrade to a bigger droplet, but maybe other solutions?