What cause this after after server shake and upgrade?

Hello, I have one Codeigniter system used by more than 30 clients in my server, previous time was running smoothly, but few days ago my VPS server become slow due to higher load sometime range from 60/4cpu this create unusual behavior when sales person A login in that system after sometime change to sales person B without known and start sales to sales person B store. I decide to upgrade to Dedicated server but still the issue exist . I talk to that system developer and said nothing wrong to system and It's true because previous work good. So my question maybe is their any where in server affect Codeigniter system and cause user login change account automatically ?