Undeterministic Asp Failure

I use an asp page to upload a word document to the web
server (with Soft Artisans SA file-up), but sometimes it

doesn't respond, seems to return with a zero sized answer.
I use IIS 6 on Windows 2003 Server and I use this machine

as a client too. The communication goes throught ssl
channel, HTTPS by certificates. If I use Windows 2000 as a

client from another place (that case there is a firewall
between the client and the server) sometimes the same

incident happens, but the Internet Explorer shows
the "Cannot find server or DNS error" message instead of
the blank

response. The problem happens independly of the parameters
which I fill in the form and independly of the file what I

choose. Sometimes the document uploads and sometimes not
with the same parameters. I cannot define the cause
exactly .