In relation to using sessions and fpassthru together.
Try adding: session_write_close()
somewhere near the top of the download script, before you start sending the video, and that should take care of it.
I've implemented and tested session_write_close() and it works like a dream. Other links can now be clicked and loaded whilst a big file is being passed using fpassthru.
Big thanks to Greg for this tip. What a helpful community we live in :0)