Currently I have a cron that restarts PHP-FASTCGI every 15 minutes but.....yea. 64 bit - Debian 5 nginx 0.6.32
try to install apache as backend for nginx php cgi. this solution is widly spread.
In addition to vartecs advice;
1) upgrade your nginx, 0.6.x is pretty old.
2) once php-fpm, configure /status and /ping to monitor it.
3) You might want to use nginx HttpStubStatusModul.
4) Something I've read on lightys wiki:
To limit this occurance, set PHP_FCGI_MAX_REQUESTS to 500."