Nginx - The connection was reset?

Question

I have Ngnix installed on my host at Linode. I have the config below:

server {
    listen 80;
    server_name example.co.uk www.example.co.uk;

    location / {
        root   /usr/share/nginx/html;
        index  index.html index.htm;
    }
}

server {
    listen 3333;
    server_name example.co.uk www.example.co.uk;

    location / {
            proxy_set_header   X-Real-IP $remote_addr;
            proxy_set_header   Host      $http_host;
            proxy_pass         http://127.0.0.1:3838;
    }
}

Strange is that Ngnix sometimes returns 'This site can’t be reached' (on Chrome & Opera) 'The connection was reset' (on Firefox) when I access a port via a domain name that I have already pointed to the IP address.

example.co.uk - Fails at times
example.co.uk:3333 - Fails at times

Access a port via IP never has any problems, eg.

139.160.200.50 - OK
139.160.200.50:3333 -OK

What are the reasons for this strangeness?

Any ideas?