I think Hyperterminal must be lacking the proper functionality for high speed communications, but I don't know why. There are many many other DNC utilities out there that will do the job properly. I started out using a simple program called Shadcomm (from the Millwrite engraving people), but now that I have OneCNC, I use their utility.
I think OneCNC had a free to try DNC utility somewhere, with a nag screen , but I cannot find the link to it at the moment. You could contact the Florida office maybe they'll have a link for you.