Need fix for my VPS crashes

The problem: Recently my server started crashing over and over. There weren't any crashes for 3+ weeks and after an update of "WP super cache plugin" I had to manually edit my htaccess file to apply some rules. On the next morning occurred the 1st crash. Then I removed the plugin manually with every file and directory from the FTP + edited to the original position the htaccess file. 30+ hours after that occurred the 2nd crash. Haven't done anything after that, just rebooting the server when crashes and for the last 4 days there were 4-5 crashes The server: It hosts only 1 site with under 100k monthly visitors, but increasing. The hardware of the VPS is 2x2.4ghz CPU, 4gb RAM, 80gb SSD. This is self-managed VPS. Attaching screens from the console and the httpd and MySQL error logs so you can get a better idea. More server details - Nginx, Centos, MariaDB. It also has a created swap file. The VPS provider is [login to view URL] The website: The website hosted on the server is a free service website which works with a javascript form, which calls a PHP function with an API communicating with another server. All of the users are using this form when entering the website. Its platform is Wordpress. Before I transfer the website to this VPS it was on a shared hosting. I had to transfer it because they have limited it to 15 PHP processes which weren't enough for the website and was showing different server errors for insufficient resources. What I want: Identification of the problem and a permanent fix for it in 12 hours + stress test + a report what has been done. I was working with 2 more freelancers on this VPS, 1 of them was setting the server + transferring the website and the other one I hired 2 weeks after to optimize the server and create a swap file. As I said before - after the optimization the server hasn't crashed for 3+ weeks and according to my google analytics, I had few days that were even hotter than these 3 days of crashing. Because of this, I want 2-3 weeks guarantee that the problem won't appear again - a PERMANENT fix of this problem. My budget: It is a fixed price of 50EUR. Of course, the freelancer system gives us the option to set custom bids and I reserve my right to take whatever bid I see more suitable for my needs. P.S. Please, excuse me for my English as it is not my native language.