502 Gateway Errors under High Load (nginx/php-fpm)

This should fix it…

You have:
fastcgi_buffers 4 256k;

Change it to:
fastcgi_buffers 256 16k; // 4096k total

Also set fastcgi_max_temp_file_size 0, that will disable buffering to disk if replies start to exceeed your fastcgi buffers.