Are you running multiple versions of PHP?
If so, it might be a different version of PHP-FPM that you need to restart.
You also might want to change the connection mode from “socket” to “tcp port”. On a few of our older servers we found the “tcp port” method was a bit more stable.
System Settings > Server Templates > Default Settings > PHP Options
*** I’m not sure if this change will address existing domains, or only new ones. ***