Microcache will offer a WSOD after a while

I'm serving cached version to all anonymous users. I'm kind of using it as a lazy man's varnish drop-in, it works great except it will offer a nice WSOD from time to time, and exclusively for the website.com front page.
I don't know why or when, but it looks like it depends on how busy the site is. The more anon users on the site, the faster the frontpage will wsod.

I've not found any errors in nginx or php logs, http response is 200.
Where in microcache configuration should I start looking?
I'm using Perusio's nginx config, nginx/1.2.5, pressflow6

fastcgi_microcache_zone.conf

fastcgi_cache_path /var/cache/nginx/microcache levels=1:2 keys_zone=microcache:5M max_size=1G inactive=2h loader_threshold=2592000000 loader_sleep=1 loader_files=100000;

microcache_fcgi.conf

fastcgi_cache microcache;
fastcgi_cache_key $host$request_uri;

fastcgi_cache_valid 200 301 60m;
fastcgi_cache_valid 302 3m;
fastcgi_cache_valid 404 10s;

fastcgi_cache_use_stale error timeout invalid_header updating http_500;
fastcgi_ignore_headers Cache-Control Expires;
fastcgi_cache_bypass $no_cache;
fastcgi_no_cache $no_cache;
add_header X-Micro-Cache $upstream_cache_status;
expires epoch;
fastcgi_cache_lock on;
fastcgi_cache_lock_timeout 3000; # in miliseconds.