Might be caused due to a ill programmed plugin or any abrubt process utilizing php.
Usually its not hard to figure it out.
After changing settings, restart your instance once.
Then state if the problem is persisting.
You may also look to configure additional setting in both php-fpm and nginx.
For php-fpm, its always advisable to go from the bottom values and increase it gradually according to your resource, I tell this to people, rather than using general values as because everyone’s user case and implementation varies greatly hence its good to find out what values work for the best.
Meanwhile check in logs for what process is exactly consuming the most resources.
You can get many tools to do so. I use netdata overall to have glimpse on the resource utilization.
setting pm = dynamic or ondemand might help.