IIS7 - Specifying content-length header in ASP causes "connection reset" error

Question

I'm migrating a series of websites from an existing IIS5 server to a brand new IIS7 web server. One of the pages pulls a data file from a blob in the database and serves it to the end user:

Response.ContentType = rs("contentType")
Response.AddHeader "Content-Disposition", "attachment;filename=" & Trim(rs("docName"))&rs("suffix")' let the browser know the file name
Response.AddHeader "Content-Length", cstr(rs("docsize"))' let the browser know the file size

Testing this in the new IIS7 install, I get a "Connection Reset" error in both Internet Explorer and Firefox. The document is served up correctly if the Content-Length header is removed (but then the user won't get a useful progress bar).

Any ideas on how to correct this; whether it be a server configuration option or via code?

Edit 1: Did a bit more trial and error. The requests will succeed if both "Enable Buffering" and "Enable Chunked Encoding" are false. If either one is enabled the error occurs.

Edit 2: More trial and error testing; turns out that text files will work fine with the script; only binary files (images, pdfs, etc.) will fail. Still completely clueless otherwise.

Thanks.