PHP/XML - session time?

Hi,
What I do: I want to export huge amount of data from MySQL into XLS
file using XML format via website. It works like this: on website
there is link do php file which contains XML code with MySQL querys.
What's working: Working when I download this dynamically generated
file on very fast connection +1Mbit/s
What's not working: When I try t download the same file on slow
connection (for ex. 512kbit/s) then the file is corrupted, and when
try to open this in excel i get error that not every XML tags are
closed.
The file size is about 5MB.

Maybe someone know the solution or where to look for it. I don't have
full access to webserver, and I can only change my code or data base
(MySQL and phpMyAdmin). I cannot change any php.ini or anything from
configuration files of webserver or mysql system.


Best Regards,
S³awek