[solved]File Downloading cuts/brakes prematurely

-- url of the page with the problem -- : mywebsite.com/mypage
-- HikaShop version -- : 2.2.2
-- Joomla version -- : 2.5.18
-- PHP version -- : 5.4.24
-- Browser(s) name and version -- : any

I sell digital goods of about 800MB, 1.5GB, or 2GB (.mp4 movies)
The problem is that when a customer tries to download his paid file then the downloading process cuts randomly (or brakes) prematurely and always before it should. For example at 550MB out of 840MB ...so the movie won't play...
Of course, I already did the tests to download through direct HTTP link (http://website/file.mp4) and FTP (ftp://website/file.mp4) and there is no problem whatsoever.
The server is a quite capable dedicated machine with plenty of resources to store and serve big files but maybe this information helps: while uploading he needs the FTP client (Filezilla) to send "keep-alive" requests ...I'm not sure why but it works fine like this.

Any thoughts about the downloading problem? ...(it's a bit urgent )

Thanksss!!

Right next to the value of max_execution_time PHP Configuration Editor (cPanel) says: "This sets the maximum time in seconds a script is allowed to run before it is terminated by the parser. This helps prevent poorly written scripts from tying up the server. The default setting is 30.

Is there any workaround? ...I could set this the value at any number ...just to see if it helps but it doesn't seem like the best idea.
Right now I'm rebuilding Apache+PHP changing some cache managers and other handlers. Am I the only one with such problems?

Also (as expected) this method of serving downloads doesn't support Download Managers and resuming(!), this would obviously be very helpful for the customers and for our peace of mind.

EDIT: maybe the main problem is gone! ...I'l do more tests to be sure...

What I did, changed Apache to run PHP from fastcgi to mod_ruid2 [the magic of a DSO handler(?)]
That fixed another crazy thing, simultaneous downloads(!) ...only 1 download was supported before the fix (was throwing internal server errors) ...but no Download Managers or resuming