Sys ad / php question regarding 502: Bad Gateway error

hi all... a little off topic an a little on topic... i have an app in php4, running on FreeBSD/Zeus. When the browser makes a request for one of my pages (.shtml with php code) it sometimes - not always - returns a Error 502 Bad Gateway error message. i have looked at the docs and found some not very helpful stuff on the 502 message, and i am wondering if improperly set cookies, wrong settings in the php.ini, bad database connections or something to do with php/mysql rather than the webserver or OS is the problem. this is what common error codes says about the 502: The server, while acting as a gateway or proxy, received an invalid response from the upstream server it accessed in attempting to fulfill the request. any and all help is appreciated!