Readfile problem with Norton Internet Security

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Hi all,
As says the topic, I recently encountered problems with customers using
one of my file management applications and being protected in the same
time with Norton Internet Security firewall.
When the Security is not activated, downloading files works fine, but as
soon as it is activated they can not download anything bigger than 500kb !
I checked in the source code, and whatever the method to read the file
on the server (readfile, include, file_get_contents, fopen then
fpassthru), the file is accessed correctly but not sent to the browser
if it is over 500kb ! The headers are fine.
No cache functions (no ob_start and co), and even with them it does not
Maybe there is something to change in NIS but I tried everything and
could not find it !
Any idea appreciated

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