Apache's mod_fastcgi has a problem (?) with non-blocking socket polling (which is turned on by default with a timeout of 30 seconds). This materializes in getting random (but quite often) internal server errors. If you look in the logs you will see how mod_fastcgi times out. The solution is to add '-appConnTimeout 0' to the 'FastCgiExternalServer' Apache config directive, which will cause mod_fastcgi to open the socket in blocking mode. This has no performance impact and the Apache+mod_fastcgi+ZServer combination becomes rock-solid.