Random 504 Errors Nginx Linux Plesk

Currently have about 30 websites on Linux Ubuntu with Plesk Onyx. Something is causing random random 504 nginx error which affects ALL website on the server. Sometimes the server with recover but if I see it I restart "Reverse Proxy Server (nginx)" which allows the websites to start working again. Most of the websites on the server are Joomla based. Attached is a image of the CPU in Plesk (Health Monitoring Extension) you can see the times the server is not working.