Why are mysqli_cached.php errors pouring in?

Question

MySQL

Good day!) There is a site on opnecart3 and vds on ubuntu with vestacp, they set up memcache and other caches on it, in general, now all sorts of shoals like lack of memory and so on get out. All this happens randomly, and I noticed that on the page with a list of products, when you try to sort them, change the pagination page, etc., the products either load quickly or incredibly slowly.
And from time to time such messages drop out:

In addition to them, there are also these:

Previously, all this happened very often, and server errors 500 also periodically fell out, until I set such parameters in the vest,

it became a little better, but it’s still simple, climbing the goods you can easily catch something like "error no 2006", Can you tell me what settings you need to set and what do you advise?