Priority on Specific applicaton

Hi,

I have a CentOS machine running a Websever, FTP Server and an Telephone
system (Asterisk) on the SAME machine

on My Router, I setup the port [ 8500 - 45000] with high priority for
the voice
Which is OK... until somebody download a file. (via HTTP)

As far as I know, the download of HTTP is on port 80 (eventually 8080)
Which shouldn't affect the port range [ 8500 - 45000 ]

I would imagine that the problem is NOT on the QoS -> Router, but I just
wonder if the Linux Machine (pentium 4, 1 GB RAM) is just 100% load

Is there a way to prioritize the ASTERISK application (service) than HTTPD ?

Thanks for your help
.