Using the download control and I get an "HttpException" from w3wp.exe "Cannot redirect after HTTP headers have been sent."

1. Happens on production IIS 6 server and not development IIS 7.5.
2. Happens with a 450Mb wmv file but not smaller PDF files.
3. The Request URL in the exception repeats the base URL over 80 times then the file name a similar number of times.
4. Using a 256K buffer

Any thoughts? I am trying to avoid setting up an IIS 6 development environment.

Thanks!
Chris