Getting a download prompt due to a response mixed with two requests

I have 4 iframes in my page, whose src changes as I scroll down my webpage. The faster I loop through the src, the more often. If I scroll fast enough I get a download prompt to FF.

The uploaded file contains garbled text, followed by HTTP response headers, and then more garbled text. When I commit this on air I see that the browser wrote the request headers before one of the responses was complete, and then the server completed the response for the unfinished response, and then the response headers followed by the intermediate request response.

This happens in IE as well, instead of getting the download prompt, I get garbled text in one of my iframes.

Where is the error on the client side or on the server, how to fix it?