Flush can be interrupted by the web server you are using. Most commonly having GZIP turned on will cause the output to complete first before sending the whole thing in a compressed format. It could also be the server itself such as some older Windows servers.
You do not strictly speaking need the Output Buffer portion of your code. For what you are doing it is not needed. Flush() should be sufficient.
If you skip ahead in this tutorial to "Works with gzip" you can find ways to resolve your issue. (thanks jmbertucci)