What are the sites running? A content management system like WordPress? You may need to put some FastCGI tuning in your relevant php-fpm user.conf file, or in the vhost conf -- increasing various timeout settings.
Per-user tuning available under that version of php-fpm:
/opt/alt/php-fpm83/usr/etc/php-fpm.d/users/username.conf
pm = ondemand
pm.max_children = 4
pm.max_requests = 4000
pm.process_idle_timeout = 15s
;listen.backlog = -1
;request_terminate_timeout = 0s
rlimit_files = 131072
rlimit_core = unlimited
catch_workers_output = yes
Additionally/alternately, you could tune the particular vhost conf to include various FastCGI parameters that could help mitigate your issue. Here is what I have on a demanding Drupal site under Nginx (especially the buffers):
fastcgi_pass unix:/opt/alt/php-fpm82/usr/var/sockets/username.sock;
fastcgi_split_path_info ^(.+?\.php)(|/.*)$;
try_files $fastcgi_script_name =404;
fastcgi_index update.php;
include fastcgi_params;
fastcgi_buffers 16 16k;
fastcgi_buffer_size 32k;
# Block httpoxy attacks. See https://httpoxy.org/.
fastcgi_param HTTP_PROXY "";
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
fastcgi_param PATH_INFO $fastcgi_path_info;
fastcgi_param QUERY_STRING $query_string;
fastcgi_intercept_errors on;