Some users can't fully download the file

I’ve got a PHP script which allows users to login and download some files. It works almost fine, except that some users can’t fully download some files. Some user claims that he can download a 40MB file, but fails to download a 50MB file. Sometimes it stops downloading at 33MB, sometimes at 45MB, it’s not a constant value.
I’ve heard that Mod_secure can cause that, but disabling it didn’t help.
The download lib which I use is here

So what could be the reason of this issue?