Dear Abe. Thank you for your advise. I tried to disable plugins (WPBakery Visual Composer, rtMedia for WordPress, k-elements, Taxonomy Metadata) and yes the problem completely dissapeared. Visual Composer in your theme dramatically increase the server load and cause mysql server to stop.
I have another WP website with Shadow theme that is using Visual Composer at digitalocean.com with the same server environment, but only website with KLEO theme experience such a problem.