Internet connnection speed is slowing down while surfing and downloading on Fedora 11

[student8185]

Hello everybody

I am using fedora 11 now, and I see that while surfing (by firefox) and downloading (by download managers) the internet speed is slowing down and some of my command line installations (using yum) are failed due to time out errors. The speed is going up and down repeatedly.(Average speed is slow)

I am connecting to the internet by connecting to a LAN in my workplace and then using a VPN(Virtual Private Network).
I connect to LAN using wireless modem whose driver is newly installed from its manufacture.

I have Windows 7 too, and its connection speed is very good.(for both surfing and downloading)

I don't set any proxy server for my connection(in both Windows and Linux)

Before fedora, I have tried Ubuntu(8.10, 9.04) and this problem were exits.

What is the reason for this problem?
How can I solve it?

And I don't know where to ask my question, but I think this is related to network.
Note I am a newbie.
Please help me step by step.