APACHE, PHP Server return randomly empty response

问题

I'm having 9 ubuntu servers (EC2) working behind a ELB, The servers seem to response a empty response randomly even if the request is send not from the ELB,

No error log was captured on the Apache monitor, No special timeout, or 408 is captured on the access log, Both return status 200,

The only thing i can see different between a request with a response and a request with an empty response is the headers which:

Good Response headers:

Cache-Control:      
no-cache, must-revalidate
Connection:     close
Content-Length:     3 KBytes
Content-Type:       application/json
Date:       
2014 Oct 10 17:41:39-15s
Expires:        Mon, 26 Jul 1997 05:00:00 GMT
Server:     Apache/2.2.22 (Ubuntu)
X-Powered-By:       PHP/5.3.10-1ubuntu3.13

Bad response headers:

Connection:     close
Content-Encoding:       gzip
Content-Length:     20 Bytes
Content-Type:       application/json
Date:       
2014 Oct 10 17:41:26-13s
Server:     Apache/2.2.22 (Ubuntu)
Vary:       Accept-Encoding
X-Powered-By:       PHP/5.3.10-1ubuntu3.13

Any idea?

来源：https://stackoverflow.com/questions/26325120/apache-php-server-return-randomly-empty-response