blank page on some micro-cached pages

[Ronald Van Assche]

Running Nginx 1.5.7 on Freebsd 9.1
with php-pfm 5.5.3

a wordpress site is "micro-cached" , but for some strange reason and not always , some pages ( wordpress post or pages even 3 or 7 days after their publication)  are rendered for some users ( me and others and not always the same, with different browsers) as blank pages and the same connexion can read other cached pages/post with no problems.  

The size of the global site in /cache is 72 Kb. Very small indeed.

The bad cached pages are logged like this :
"GET /url  HTTP/1.1" 200 31

always 200 as a result code, and 31 for its size.. 31 is not the actual size of the page of course.

If a refresh my local cache ( SHIFT CMD-R) on chrome , the correct page is reloaded and cached in /cache/x/y/somefile by nginx.

Any help ?

Some config :

fastcgi_cache_path /cache/nginx levels=1:2 keys_zone=microcache:5m max_size=2000m;
sendfile on;
sendfile_max_chunk 512K;
aio sendfile;
tcp_nopush on;
read_ahead 256K;

open_file_cache max=1000 inactive=20s;
open_file_cache_valid 30s;
open_file_cache_min_uses 2;
open_file_cache_errors on;

keepalive_timeout  120;
keepalive_requests 10000;

client_max_body_size 2m;

 gzip  on;
 gzip_buffers      48 8k;
 gzip_comp_level   4;
 gzip_http_version 1.0;
 gzip_vary      on;
 gzip_disable "MSIE [1-6]\.(?!.*SV1)";
 gzip_proxied    any;
 gzip_types    text/plain text/css application/x-javascript text/xml application/xml application/xml+rss text/javascript;