I've tried before and today I decided to give it a new go.
I'm running Windows Vista x64 Ultimate on my pc.
I only have 1 HDD in my pc but maps some network drives for added space.
These are running in a FreeBSD machine running Samba.

From within XP Pro I can use these as download path or just like any
internal hdd without any problem.
When booting Vista and trying to download some BitTorrent traffic to these
mapped network drives it will fail direct it reach som higher speeds. I got a
100Mbit connection and when downloads get above some MB/s it will stop and
the mapped network drives will be shown as inaccessible. This can be fixed
with just restarting the torrents but it will soon happen again if they reach
higher speeds.

Just for a test I tried download from the FTP-server on my server to the
mapped drive and that worked fine @ ~14MB/s. So it seems that it's just
Bittorrent traffic that's causing this problem.

Any suggestion to this problem is greatly appreciated!

PC Specs:
C2D E6600 2,4GHz
2048 DDR2
Abit AW9D
Windows Vista Ultimate x64

After re-installing vista and trying out diffrent solutions for a couple of
days now I think I found the problem.

The problem was the driver for the NIC Windows Update updated. Ignoring the
update and/or installing the driver downloaded from Realtek prevents the
error from happening. If someone else out there having the same problem hope
this information will be helpful.