Vista Drops mapped drives

I have a Win 2003 domain based network which has been running successfully
for 3 years now. I have now attached a Vista Business based workstation. I
have had a problem with mapped drives, which are set up by a logon scripts
based on the server (which again has been working successfully for XP based
workstations for 3 years).
On the Vista machine after periods of inactivity the workstation loses
contact with the mapped drives. These drives still appear in the 'Computer'
windows but are marked with a cross although sometimes they appear correctly
mapped but when you try to access them a message saying unexpected network
error occurs. Drive mappings from the XP workstations are not affected in
this way.
However, if I go through the 'Network' window and access the data that route
then I can get to the data so it is not a network problem per se. If the
network was not functioning (or indeed if it was the network card sleeping -
see below) I would not get access to the same folders as the dirve mappings
would I. Or if it was a sleeping network card then waking it up via the
'Network' route should wake it up for the mapped drives option as well
shouldn't it?
I can also in the 'Computer' window map a new drive and have that work
correctly.
I have followed other threads suggesting that it might be a network card
power down so I have disabled the ability for the computer to shut down the
network card.
I can reset everything by powering off the machine and restarting but at the
next period of inactivity the problem reappears.
Any clues would be useful. If further information is required I can provide.
Thanks
Steve