User’s mapped network drives disappear.

We have about 250 users that access folders on our file server. The most common ones get mapped at logon in a script by group membership. These mapped resources will disappear and we have resorted to manually mapping them from the user’s workstation. It seems to be getting worse with multiple daily occurences now.
The file server is WIN2K ADV Server SP4 with all updates .
It’s a mystery to us so far.