I have a forums which runs on a dedicated server.

My forum's pageload speed is file, but the attachment download speed is around 5Kb/s on a 7.2Mb/s 3.5G USB Modem.

I run php as dso, may that cause any problem? I tried using suphp and cgi, but that increases the server load and hung it.
My webhost changed the network cable, switch and port on my request, but that didn't help to solve the issue.

What else shall I do now?