Server Issues + Poor Programming = Site Keeps Crashing

Hi, I run a website that receives over 50,000 visitors a day and recently relaunched it. The new website features a custom Wordpress and Xenforo integration that allows users to be logged in to the entire site with their Xenforo account. They can also use it to comment on stories and whatnot. Basically, Wordpress is "embedded" within Xenforo. A programmer I hired from here worked on the integration for several months. Unfortunately, after the new website went live, we started to experience many slowness issues, crashing, memory leaks, dying PHP, and errors. Almost every time I post a news story, the site crashes and dies for around an hour. I believe the problem is a combination of some poor coding here and there, some heavy features that were removed that are still running, and server misconfigurations. I need to hire someone who is a highly experienced PHP programmer and also a highly experienced server administrator. You can't just be one, you must be both. We have tried having people help us who only have experience in one or the other, yet our problems still persist, so it's become very clear that we need someone who can diagnose both programming and server issues and fix them. Our server is Debian/Linux, runs with Nginx, and also has some sort of Google cache Nginx thing built in. It runs behind Cloudflare as well (a Pro Account), which I will give you access to. If you do not have heavy experience with these, please do not apply to this project. Wordpress is coded in PHP and also uses Timber/Twig for templating. Xenforo is all in PHP. Both have mySQL databases. Again, both Wordpress and Xenforo have been integrated together through a custom plugin or two that I don't completely understand. However, since I was with the programmer throughout the entire process, I do know where things are and what the purposes for most things are. So I will be here to help explain what's going on. We copied the website over to a new server where the person I hire will need to configure the server correctly, investigate what's causing all our issues, fix them, and improve anything that can be improved. You will need to be able to go through all the code, measure what PHP processes are slow or broken, test the server under load, see what's causing our memory leaks and PHP to keep crashing, etc. etc. I imagine we will first need to go through the code, then we can address the server/setup issues. My previous server admin will also be on hand to answer any questions about our setup. The basic goal of this project is to get the website working speedily without any errors, crashing, or slowness issues. There are an umbrella of skills you will probably need, so again, please only apply if you know servers and PHP programming like the back of your hand. Thank you very much.