VBScript program loses network access

Does anyone know what might be causing the following problem on Windows
Server 2003 SP2..?

I have a VB script program (Controller.vbs) that's spawned from a C#
Windows Service, using System.Process 'cscript Controller.vbs'.

The owner Domain User Id of the Windows Service has access to a network
share (mounted as G, to which output files are written.
(Authentication is with Active Directory).

The Controller.vbs script does this:

* Loop forever
- Run a .BAT script on the C: drive (which outputs log information
to the G: network drive)
- Check the G: log file the .BAT script wrote to, to verify no errors
occurred[*]
- Do some other processing and write output using
FileSystemObject.WriteLine to another file on the G: network drive[*]
- Close all open files
- Sleep for 5 mins
* End Loop

What I'm finding is that everything works fine until 7:30am each
morning. At this time, the .BAT script that's invoked from the
vbscript can *successfully* write to the G: drive. (If I check the
files myself I can see they're being created correctly).
However the .VBS script itself cannot write to or read from the G:
drive itself (so lines[*] are failing).

If I stop and restart everything, it again works fine until 7:30am the
next morning.

>From checking the Security log in the event viewer, nothing strange
appears to be happening.

Does anyone have any thoughts as to what may be causing the problem?
Our system administrators deny everything..

Thanks
Phil