500 Internal Server Error after 37-39s on PHP

Hi all, I am using Zen Cart 1.5.1 with Back in Stock 3.2 module on Godaddy VPS. Each time I do the send mail to customers (over 5000) Running each 39s the server will send me this error 404 page Internal Server Error The server encountered an internal error or misconfiguration and was unable to complete your request. Please contact the server administrator at webmaster@abc.com[/EMAIL] to inform them of the time this error occurred, and the actions you performed just before this error. More information about this error may be available in the server error log. Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request. I had try to login to the cPanel WHM setting the PHP.ini run time from 30 to 60, the result still same. And on /logs I am not see any xxx-adm-xxx.logs on ZC folder. Same cPanel server, same PHP. But on old ZC 1.3.9h with Back In Stock 3.0.2 It work ... I don't know why on ZC 1.5.1 with Back In Stock 3.2 will not work now! And I see on the ZC 1.3.9h with 3.0.2 It URL is different after run the send mail to subscribed back_in_stock_notifications.php?securityToken=d3a9ede92d119a88d5997669e5d04187&a ction=send&option=4 On ZC 1.5.1 with 3.2 It URL just back_in_stock_notifications.php?action=send&option=4 Anyone know what is wrong on it? So now I am lost the direction what is wrong on it. Any one had experience this before?? What will make the PHP and cPanel 500 Internal Server Error? Security on cPanel or something? Thank you.