Vista “loose” connection to 2003 server

We have upgraded our computers and at the same time use Vista.
I add a mapping to a share on our server, workes fine and quick.
But after a minute thoose shares get red crossed and not active for our programs.
If I open a share from my Computer it gladly opens and the green network is back, for yet a minute when it, kind of, disconnects.
I have removed the power saving for the NIC.

Why is this and how to solve it?