Network freezes /crashes when it's heavly used on Windows Vista

Hi

I have problems with several computers in my company that runs Windows Vista
Enterprise both with and without SP1.
The problem is that when a computer is genereting a lot of trafic on the
network card, it suddenly stops function.
The only way to restore network connection is to restarte the computer.

When the problem occure you will only manage to ping localhost and your own
IP addresse.
If you ping another server or computer you get "destination host
unreachable" and "reguest timed out" in random order.

Computers are mainly Dell D630 and D830, and the problem occur both on
wireless and wired networkcards.

The way I have managed to provoke the error is by copying three 3Gb files
from another computer/server and have installed Citrix Secure Access Client
for Vista.
Therefor i thought i might be the Citrix client that is the source of the
problem, but the problem still exist without the Citrix client, but it
dosen't happend that often.

I tried to disable the multicore option in BIOS, so the laptop only uses one
core. This seemed to help, but this could be because it dosn't push the
networkcard hard enough.

When copying this three files the resource Overview in Vista says it
transfers with 95-96 Mbps with multicore enabled
When copying this three files the resource Overview in Vista says it
transfers with 65-70 Mbps with multicore disabled

Following steps have also been tried without luck:

Antivirus has been uninstalled
Windows firewall has been deaktivated
Disabled autotuninglevel
Unchecked IPV6 on networkcards
Installed newest networkdrivers from Dell.
Clean install of Vista Enterprise with SP1