Currently I have a cron that restarts PHP-FASTCGI every 15 minutes but.....yea. 64 bit - Debian 5 nginx 0.6.32
Use PHP-FPM instead of standard FastCGI. FPM works great with nginx.
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:
"This problem seems to stem from a little-known issue with PHP: PHP stops accepting new FastCGI connections after handling 500 requests; unfortunately, there is a potential race condition during the PHP cleanup code in which PHP can be shutting down but still have the socket open, so lighty can send request number 501 to PHP and have it "accepted", but then PHP appears to simply exit, causing a 500 return from lighty.
To limit this occurance, set PHP_FCGI_MAX_REQUESTS to 500."