504 Gateway Time-out during POST

[David]

I have done all that you asked.

Very, very, rarely that script will execute (it's just sending a private message
to another user) as it should - basically instantly.

I see sometimes fastcgi is logging the 

I have just managed to recreate the 504 error and I can see no errors in my
php-error-log file.

The 504 occured here:

[07/Sep/2008:17:44:24 -0500] 504 POST
/forums/private.php?do=insertpm&pmid=101373 HTTP/1.1 upstream_response_time
60.000 msec 1220827464.227 request_time 60.003

and in the nginx error_log:

30104#0: *43481 upstream timed out (110: Connection timed out) while reading
response header from upstream, client