I have a Magento site on an nginx server with PHP-fpm. I came into this project recently with the server already set up, albeit poorly. Right now, when a user is in onepage checkout and tries to move from Billing Information to Shipping Information, or directly to Shipping Method, it just redirects him back to the cart. The cart page now has the 'estimate shipping and tax' box open but other than that no change. If you try to checkout again, its the same issue. The only way I was able to resolve the issue was by reloading php-fpm.
In addition to this, I get random 502 errors on my site from different customers. They too only get fixed with a php-fpm reload.
I would like to disable the php-fpm caching as its causing a lot of headaches for our store, but I am not familiar enough with nginx and php-fpm to know where I would go about disabling this to help stop this problem.