Slow server performance without apparent reason

We've been dealing with an issue for about 2 weeks that we're out of ideas of how to deal with moving forward.

Server setup and Application:
Google Cloud Instance, with WHM/cPanel
WordPress 5.0+
PHP 7.0
Apache 2.4

Problem:
1-5 times during the day, the server (and the website) stops responding (timeout). We cannot get the website to load, but the WHM and cPanel backend is also very very slow. The slowness takes up to 5-10 minutes after which things return to normal.

We can't even login via WHM during this period.

What we've checked:
WordPress error log. While we have some errors here, they do not coincide with the times when we have periods of poor performance.

Server performance - We have 1 website running on a 16CPU 8GB RAM machine that operates at about 8-10% capacity. No spikes in ram or cpu usage during these periods. The server appears to be idling and is definitely not being overused.

What we've done:
We've restarted the machine which fixes the issue. When we tried to restart apache or mysql via WHM, it fails if it's being done during this period of poor performance.

Questions:
What should we check? Any particular logs or things to look for? Do you need anything else from us re: server configuration, php, apache, mysql, or anything that can point out what the issue may be? Could it be related to PHP, MySQL, Linux, Apache, etc.? We're dead in the water about where to look for potential problems when we have an underutilized server with no apparent issue. Again, slow, unresponsive server-wide symptoms without an apparent trigger.

 

Thank you! @cPanelMichael -->

Support Request ID is: 11993787