I have had this problem with Debian Etch for some time now. My site crashes every night 2 AM in the morning. It stays down and does not auto restart. So my visitors get 502 error all night. It works with a manual restart but not with a cron job. I am clueless as why this does not happen in the middle of the day when I have my most traffic. And it baffles me why Nginx does not come back up and start again after crash.
Any ideas?