randomly showing internal server error 500 what i should do to know exact problem – CPanel

My php website is showing randomly internal error 500 and after refreshing many times it works, in errors, memory allocation problem is showing…
but when i am seeing in root directory’s error_log there is no any error.
Please someone help me to identify the problem.

Thanks for any help!

Here is My php class connection code-

class mysql{

public function __construct($db=array()) 
{   
    $default = array(
        'host' => 'localhost',
        'user' => 'XXXXXXXXX',
        'pass' => '*********',
        'db' => 'XXXXXXXXX'
    );



    $db = array_merge($default,$db);
    $this->con=new PDO("mysql:host=".$db['host'].";dbname=".$db['db'].";",$db['user'],$db['pass'], array(PDO::ATTR_PERSISTENT => true, PDO::ERRMODE_EXCEPTION => true)) or die ('Error connecting to MySQL');
    $this->con->exec("SET CHARACTER SET utf8");      // Sets encoding UTF-8
}
}

And Here Is My php file-

allow_url_fopen = Off
allow_url_include = On
display_errors = On
enable_dl = Off
file_uploads = On
max_execution_time = 30
max_input_time = 60
max_input_vars = 1000
memory_limit = 256M
post_max_size = 8M
session.gc_maxlifetime = 1440
session.save_path = "/var/cpanel/php/sessions/ea-php70"
upload_max_filesize = 2M
zlib.output_compression = Off

And This is my .htaccess file

<IfModule php7_module>
  php_flag display_errors On
  php_value max_execution_time 30
  php_value max_input_time 60
  php_value max_input_vars 1000
  php_value memory_limit 256M
  php_value post_max_size 8M
  php_value session.gc_maxlifetime 1440
  php_value session.save_path "/var/cpanel/php/sessions/ea-php70"
  php_value upload_max_filesize 2M
  php_flag zlib.output_compression Off
</IfModule>

In Metrics/Errors It is Showing

[Thu Aug 30 12:30:17.252514 2018] [:error] [pid 31450:tid 139898017253120] (12)Cannot allocate memory: [client 171.61.187.181:50334] couldn’t create child process: /usr/sbin/suphp for /home/abovo/public_html/index.php

Website link is – http://abovo.in/