The page you are looking for is temporarily unavailable. whem my visitors reach over 500+ !! does anyone know how to fix?

[escavern]

i have a problem,, everytime my visitors going high and higher above 500+, sometimes i get message:
"Server error The page you are looking for is temporarily unavailable. Please try again later" how to fix this? do i have wrong in configuration?? or is there something i need to setup for high traffic? please help me,, im tired of this problem,, im googling it and doesn't solve my problem.. THX

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,77591,77591#msg-77591