On a WordPress setup when it comes to caching for improving the page load times, we strongly advise you to use Varnish cache (it comes with N99panel) instead of using a caching plugin.

If you use a caching plugin for any reason whatsoever, please be aware of its URL pre-fetch or preemptive fetching feature. This feature lets the caching plugin store page content into the cache using background processes. Issue arises if the concurrency of the pre-fetching exceeds the max allocated PHP processes value. This can lead to exhaustion of VPS’s RAM leading to sluggishness and/or other unexpected behaviour.

Therefore our advice is that either,
1. If you are using Varnish, do not use other caching plugins in WordPress.
2. Or if you still wish to use a caching plugins, disable the preempt feature or lower its concurrency and frequency.